For the record, I sometimes get that siege behavior without ice drakes (haven't actually seen any of those yet). I think (but am not sure) that it's been the vamari every time; I figured it was normal behavior (i.e. that they were actually sieging me, instead of suicide charging me). They ride war horses, which don't have any problem being ridden usually, and they definitely attack normally sometimes.
I'm getting a pretty strange bug, though--my dwarves and pets keep climbing up onto places where they shouldn't be able to get to, often starving or dying of thirst there. It seems to usually happen in high traffic areas, like statue gardens and next to busy hallways. I just had a child die on the roof of one of my original seven's house; there hasn't been access to that spot in years.
This seems like a DF bug, except I can't find any mention of it. I'm guessing it might have something to do with the emerald wasps, like somehow they're causing other creatures to move up a z-level when their paths intersect... or maybe there's just something funky with my copy of DF. Could be that I put some temporary access to these places, took them down and forgot about it. I has anyone else has observed this.
At any rate, the wasps are kicking my ass; it's like a catsplosion except the cats fly. Maybe shorten their lifespan? One thing that is cool: for some reason they love to hang out on random roofs and ledges that only they can access. Very wasp-like.