...as soon as a saddle is put on? once he's cinched up he'll twitch maybe anywhere from 5-10 times and then nothing else. I've had cinchy horses before, some so bad they'll run you over without a care in the world, but this is odd because he's never really done this before. especially since he'll do it with the saddle just sitting up there and everything still slung over the saddle.
There was a lady at the barn I board at whose horse would twitch when she put on her saddle. Turns out the saddle was pinching one of her nerves so bad she would twitch all over.