Why did my angle grinder grab like that?

Yeah, that combination you described is plenty to cause a bind. A cutoff wheel is thin and has basicly zero tolerance for the cut closing up on it. If the bolt coud rock in the vise even a little, the slot pinches the wheel as you cut, the wheel grabs, and the grinder torques out of your hands. Cutting at an angle makes it worse becasue the wheel is loading one side of the slot instead of sitting square in it. Fix the workpiece first. Shim it, wedge it, clamp it twice if you have to, so nothing moves at all. Then cut square with a light touch and let the wheel do the work. If you're pushing hard enough that sparks slow down or the wheel tone drops, back off. A binding wheel on a grinder will walk right through a leather glove in a split second, which you found out. On the gloves question, I wear them and most guys I work with do too, but the argument you're worried about is real. A glove that catches on a spinning wheel or burr can pull your whole hand in before you can let go, where bare skin might just get a graze. The middle ground most people land on is a snug fitted leather glove, no cuffs, no loose fingers, nothing that can snag. Skip the big floppy winter work gloves entirely around a grinder. And keep your thumb off the back of the wheel area, that's where a kickback ussually puts your hand. Thin vs thick wheels: not in your head. Thin wheels remove less material so there's less side load to grab, and they cut faster with less pressure, which means less of that grabbing force to begin with. The tradeoff is they're more fragile and can shatter if you twist them, so they forgive nothing. For bolts and hitch work a 1mm or so cutting disc is what most people use. What matters more than thickness though is wheather the workpiece is solid, because no wheel survives a closing cut no matter how thin it is.

One thing to add, check that disc over good before you use it again. Any wheel that's taken a hit hard enough to yank the grinder out of line can have hairline cracks you won't see until it lets go at speed. I had one come apart on me about a year back after a bind, chunks of it buried in a plywood wall a good ten feet away. Two bucks for a new disc versus that, easy call. Also worth figuring out wich way your grinder will kick when it grabs and standing so it swings away from you instead of across your body.

Same wake up call happened to me on a fence post a while back, and the thing that stuck was body position. Now I always stand so the wheel plane isn't pointed at my gut or my legs. If it grabs, the grinder swings off to the side instead of into me. Took one kick to learn that. Honestly for small bolts I mostly reach for a hacksaw now anyway, slower but it never tries to kill you.

Reading this with my own grinder sitting in the garage and feeling a little nervous now. I've only ever cut a few things with mine and I don't think I paid any attention to whether the piece could move. What I keep wondering is how tight is tight enugh in the vise, like does it need to be squeezed down hard or just held from rocking? I worry about cranking it so hard I bend whatever I'm cutting.

Update: took the advice about body position and stood off to the side of the wheel plane for the rest of the cuts, plus clamped the bolt down way tighter so nothing could rock. Night and difference, no grabbing at all. Still staring at that wheel that bound up though. Is a bind alone enough to toss a disc, or only if you can see or feel damage?