I'm new to fixed gear riding, and my bike only has a front brake. I have noticed that when I use resistance at the cranks to slow down, rather than the front brake, eventually the cranks will begin to loosen. I've tried thread-lock and torquing them down really tight, but they still eventually loosen up again.
(edit) It's a brand new bottom bracket and the cups are not slipping - it's the crank arm bolts that are coming loose. I will take a look at the crank arm interface to see if they are rounded off at all. It doesn't seem like it would be though, since once I re-tighten the bolts, the cranks are stable. It's only through the repeated "engine braking" that they get loose.
Is this just a thing that fixed gear riders deal with, is it a mechanical defect, or am I doing something wrong?