Help; I need it.

In the crit yesterday I was sprinting up the side of the field to move up, when out of nowhere my chain dropped off the big ring (to the outside). Besides nearly eating **** in front of a bunch of pros, and hitting my chin on my bars, I'm mostly baffled as to how that's even possible.

I wasn't shifting, hadn't hit a bump, etc. The chainring is about 3 years old, chain is much newer, but have never had a problem with it until now. Chorus fwiw.

I did see a tiny nick on one of the teeth of the big ring; I wonder if that was from the gravel stage the day before or something..

Anyway what can cause this, and wtf can I do to fix it??

I need confidence in my chain again!!