Advise needed really upset

[Whatshappeningtomydog]

My dog is 12 he's fit and healthy although would be considered pretty old for his breed he's exceeding the average age for the breed by 18 months. He's always been brilliant with other dogs, in his home, in his bed, eating his food, when out for a walk etc.
But he's been attacked twice in the last 5 days and on both occasions the owners swear blind that their dog has never done it before, the one today was visibly upset. And this is not just dogs scrapping both dogs tried to kill me dog.
Today my dog (who's about 12 inches to the shoulder) and was on his lead as there were livestock around met a very large young lab off its lead who bounced over and was in my dogs face, my dog grumbled/growled (its sort of throaty grumbly growl no bearing of his teeth if that makes sense he will do this occassionally if off the lead and a huge bouncy dog gets right in his face) and the lab went absoltely berserk. My dog has been injured.
The other day my dog was off his lead and a cocker spaniel just went for him for no reason. On both occasions these dogs rolled him onto his back and were ripping at his throat. Today I was quicker to act and just kept repeatedly kicking the lab (who had no collar of for me to grab) and as mine was on its lead I could get him behind me but even then he repeatedly lunged at him as if to to kill him. Also on both occassions my other dog, who today was on the other end of one of those double lead things, was totally ignored.
Why is this happening? Is it coincidence. Is it because my dog is now old that other dogs see him as vulnerable, an easy target. One final thing on both occasions we were walking with a dog we don't normally walk with that much, his usual walking partner is away. This dog is nervous of other dogs (she was a few steps behind my dog) whereas his usual friend is exceedingly bouncy and friendly to other dogs and would rush over to meet anything probably ahead of my dog.
At his last vet check he was found to be completely fit and healthy and barely showing any signs of old age, the only obvious thing is that he's greying and his eyes are clouding over. He doesn't have a squashed face and he has a gorgeous thick tail long tail generally carried fairly high but not over his back etc so I would have thought there was little problem for other dogs to read his body language.
Any thoughts suggestions will be gratefully received.

[SparklingRaspberry]

It could be scent. It could be a vibe he's given off subconsciously. It could be body language.

My dp's lab is as dopey as anything, he pays absolutely zero interest in other dogs when out yet 99% of male dogs who see him turn into demons towards him! It's always males never female. Sometimes they're on the other side of road sometimes even further. The thing is my dp's lab couldn't care less and the owners are always shocked and swear their dogs don't usually do it and you can always tell what ones are telling the truth.

The vet and behaviourist both reckon it's his presence and scent.

It might be worth asking next time (if) it happens whether they're male or female.

[whatwouldrondo]

It sounds to me as if your dog is defending the nervous one and effectively telling the other dogs to F off. Dog etiquette is fairly subtle and perhaps your dog is not very good at trying to be alpha as a result of lack of practise, not really being in a position to be an alpha because of age and other factors, or just being a grumpy old man......

My dog did this, not at all agressive or dominant normally, certainly not an alpha dog but as he got older he could snarl at any dog that tried to be dominant with him which could then lead them to attack. He went to a kennels that also did training and behavioural help and they said exactly that. He was being defensive, like an old man telling a youngster to fuck off which could bring out the worst in younger dogs. It was worse with breeds who had previously gone for him or after he had had a bad experience, and certainly when he was with a friend's dog who he had grown up with. I dealt with it by bringing him to heel when certain breeds were approaching so he knew I was in control, black and brown labs were actually one, and asking the owners to keep their dogs away (with some excuse about him being frail), distracting him from confrontations, and generally building his confidence in his interactions with other dogs so he did not feel threatened.

[Whatshappeningtomydog]

Both dogs were male dogs. My dog is uneutered as is my other one although as I say he's been totally ignored on both occasions. I suspect the lab today was uneutered. It was a very beautiful very virile looking young male, and I would say very tall for a lab but also heavily built. It sort of ran over like a young dog does, was in his face but it was not giving out unfriendly signals at first. Intially I had no concerns at all I thought it would be the usual dog sniffing tail wagging nice to meet you sort of thing.
I guess being on a lead might have made him feel more vulnerable especially when you've got a 70lb+ dog running at you and perhaps he felt he also had to protect my friends dogs my other dog his friend who was on the same lead, who again is very well socialised with other dogs but if intimidated is a complete wet blanket curls up and dies. We were also in quite a narrow part of a field I'd just turned a corner and didn't see the lab coming. So options to escape were very limited.
I understand you can get special harness which warn other owners to keep their dogs away is that right?

[Whatshappeningtomydog]

Thank you by the way for the advise. He's been to the vet and is looking rather sorry for himself. He wont be out running around for a few days which is a bit of a relief. His young friend boisterous pushy rushing up to all dogs we meet friend is back at the end of the week.

[Sprockermum]

My sprocker has a tendency to suddenly and unexpectedly "have a go" at random dogs so I call him to heel until I'm sure everyone is calm and happy, so am grateful to the op's insight as to why. Hes a young neutered male so it's not just the intact dogs that do this. He's never bitten and has backed off straight away when the other dog has submitted. He's never done this to a bitch though. So maybe it is a male Alpha thing...