Okay, here's what happened.

I killed a bunny (totally justified, it was going for my wallet). Several minutes later, I accidentally fell off a cliff and died. Following my death, I expected the game to restart me at a previous save I'd made about twenty minutes before the bunny kill. It didn't. It started me at an auto-save which put me right in front of the bunny I'd previously killed. The game auto-saved at the bunny kill. Why is this? Does it auto-save after each kill (because I hadn't noticed this before) or is it something to do with bunnies?

There is a bunnies slaughtered stat so maybe

I'd be willing to bet that you opened up your menu shortly after killing the bunny, which resulted in an auto-save, as you can find in the settings that the game will auto-save at certain intervals when you open your menu.

are you on console or pc? i thought i heard consoles autosave every 15 minutes.

I'm on console. I wasn't aware that the game auto-saved at intervals (thought it was just on fast-travel, sleep/wait, and entering new cells) so perhaps that's it.

im not certain, im on pc. i just thought i read it in a thread somewhere.

The game auto-saves every 15 min or so. (Unless you manually changed that)
It's possible it was 15 min from the last save, so it auto-saved again.

I found it auto saving when I was wandering around outside a lot. Very helpful. Glad that it does three auto saves before overwriting.