I agree with my boy Tangsooyuk or whatever hahaha. As he said, when you sprinted, it pulled the wheel farther up into the track end (making the chain slack). Then later it was derailled. Check the inside of your chainstays to see if there is any tire wear on it (then that is definetely what happened).

Glad you are okay.

As for the whole running brakes/ not running brakes ****: On my track only bike I obviously don't have a brake. On my street bike I run a front brake. There is really no reason not too, you can push everything soooo much harder, and not worry about getting ****ed up. I'm not really saying that you should run a brake, just stating my observation.