I wondered if anyone could shed any light on this behaviour?

My horse is 5. I own his mother and he was born at my private little yard (this makes it sound considerably posher than it is!). I broke him in myself. So he's never been anywhere else, never been handled by other people, I know everything that's happened to him since the day he was born, so there are no traumatic experiences lurking in his past that I don't know about.

He has sweet itch, which is controlled with a fly rug and mask, so I hog him (otherwise the rug doesn't fit properly, he gets really hot, and he rubs his mane into a mess). When I started hogging he was quite happy for me to run the clippers up his mane, and then to finish off his forelock and between his ears with the scissors, where the massive cattle clippers I have won't fit. He's fine to be clipped on his body, never had any problems there.

However, he has gradually got worse and worse with having his mane done. I always approach calmly, ease him into it, take it slowly, but he's got to the point where touching his mane while holding clippers in the other hand makes him jump out of his skin. I thought perhaps they'd caught and pulled a bit of hair and upset him without me realising, so I switched to scissors for the whole thing, just to get the job done. Now he jumps out of his skin when I approach with the scissors! Yesterday I cut his mane down again and it took me an hour and a half, and I unfortunately had to twitch him to do between his ears at all.

I worked really slowly, rubbing his mane with my hand, then just placing the closed scissors against his mane, then snipping the air while rubbing him, then combing his hair with the scissors, then very gently and slowly starting to cut. He'd stand there, loose, head down, eyelids drooping, while I cut approx. six inches up his neck, and then he'd take a huge breath in, head up, eyes rolling, and lashing out at me. Backing him up and insisting he stand still seemed to help (tying him up made it worse), but he's bitten my arm, caught my shin kicking with a front leg, and when I still insisted he stand still he kicked the stone wall of the stable repeatedly, although he stopped trying to get me. I'd start right back at the beginning, touching his mane with my hand again, and eventually he'd go back to half asleep and I got another few inches up his mane, and then it'd happen again!

I can't work out what it is that's upsetting him. He's generally totally unflappable about everything. I'm not aware that he's been hurt at any time in his life, and he's fine to be clipped. If I don't have clippers or scissors in my hands I can touch his mane, his ears, his face, everything. He loves having both his ears rubbed, and his withers scratched. The only time he's behaved similarly is when he got his jabs as a foal-3 year old. He was really frightened by the microchipping and would then start jumping around when he saw the needle approaching for vaccinations. We twitched him a couple of times for it (I don't like to, but sedating got him more wound up, and it was far less stressful for him to be twitched for half a minute and have the jab quickly and quietly), and now he isn't bothered at all and we don't use the twitch any more.

Does anyone have any ideas what could be causing this and any techniques to deal with it? It's really out of character for him. Could it be the sweet itch making his skin sensitive? I'm completely at a loss, and I'd rather try to find out what is upsetting him and work through it than fall back on twitching or sedation.