I've identified one and, honestly, today, I feel like a genius.
Ok, so maybe it was just that I couldn't see it. These meltdown thingies, the new ones where he seems to be angry and just can't control it, they get worse when I touch him. Here I was thinking I was offering comfort, when really I was escalating the whole thing. Now remember, this child is not normally affected by touch. Actually, he seeks it, bangs into things and people just to get that feeling. So in his every day life he's a "touch seeker".
But, nobody said this had to be logical to me (I bet it makes sense to him). Now that I've figured this out, we're making some progress. We've also decided that when this happens he's to go to his room. I still have to take him there, and sometimes put him back there, but I think it's helping. It's not a time out per se (I've never seen those work anyway) but a place for him to espress his anger without hurting me or his sibling. I also think the small, dim, quiet space helps him to calm himself down.
So if only I could figure out why this 'over' reaction happens sometimes and not others...