Close kitten brothers - sudden howling and hissing at each other

[SubParbanMum]

My two 6 month, neutered boys are super close, always with each other, cuddling and grooming. We’ve just started to let them out a week ago and about 30 minutes ago they came rushing back through the cat flap howling and hissing with their big puffed out tails at each other. I shut the door between them for a bit and tried to reassure both separately. I’ve opened the door and sat between them as they both stare at each other.
They’ve now relaxed a bit. Any ideas what could cause this? They’re so lovely with each other (with the normal equal kitten fighting of course) so would be so upset if this changes.
thanks

It sounds like something has just spooked them. My two occasionally chase each other and pin each other down and growl, then seconds later they are fine. Cats are bonkers, I'm sure it will blow over.

Something happened when they were outside and they were scared, probably another, bigger cat. When cats are very scared they take it out on the nearest living creature, which just happened to be their brother. It's called redirected aggression. Sometimes they redirect it against their owner.

Edit - i see you posted at 11pm so it could have been a fox instead of a territorial cat. Cats Protection recommend cats stay in after dark as most incidences of being run over are in the dark due to lack of visibility.

Thank you both. I think you’re exactly right that something spooked them. I was worried and came down to check on them in the night and they were curled up together purring so clearly over it.

Probably another cat. They're very, very young to go outside though.

They’re nearly 7 months, neutered, vaccinated and had all their treatments and nowhere near to a main road. 6 months+ is in line with all the advice I’ve been given and read.

I know it's the standard advice, but we have a seven month old kitten at home and there's just no way I would let him out to take his chances against fully grown adult cats.

I see him with our adult cats and he's so young and stupid and has absolutely no concept of danger.

Exactly as Pixiedust has described. Our 2 cats, normally calm with each other, saw an intruder cat in the garden. They stared it down together then after it had run away, one of ours turned on the other. She carried on being aggressive towards her sister for a couple of weeks and we had to keep them apart. Over time, things returned to normal.
I explained it to a vet who said they’d never heard of this but the internet says it’s a thing! I’d suggest giving them time and hopefully they’ll soon be back to normal.

Sorry, seen your update and seen that they’re best friends again!

Ahh that’s so sad!! But yes, happy to report it’s like it never happened and they love each other! Thank you though