Fixed gear cranks loosening
I guess your bottom bracket is a standard square taper?  And that your bike is getting a little old in years?  The fix is to replace the cranks.

The cause is that as you tighten the nut, the square taper hole on the crank mashes up close to the axle.  But as you pedal, and brake in your case, the cranks will minutely flex in the hole which creates a tiny space so fretting can occur.  This is when minor amounts of metal are rubbed off the inside of the crank's square hole - the bottom bracket axle is normally tougher than the crank metal.

Once there's a bit of play in the square hole of the crank it will never tighten up right again.  If you're handy with tools you might be able to take some metal out of the middle of each side of the square hole, but in reality its only a delay.

Worst case is your cranks crack through while emergency-braking.

Check out your local bike co-op.  Cranks are mostly okay on scrapped donor bikes, so they should have some in spares for cheap.