Fixed Gear crank arm slips
Your lock ring is loose.

Normally the cog is quite tight from pedaling and you can do some braking with it. When you brake harder it breaks loose and unscrews until it hits the lock ring. This is what you feel as sliding. The lock ring has left hand thread to keep the cog from unscrewing completely. Now that the cog has tightened against lock ring, the process repeats when you pedal hard forward.

Now that the lock ring rattles loose, you should tighten it immediately.