Fixie problem

Hey, so I’ve recently bought my first 2nd hand fixie, but as I got more relaxed driving the bike and started trying out skids, I’ve noticed a problem.

When I try to skid, rear wheel does’t lock but instead I feel the pedal arms a slip and loosen. Then when pedaling again I feel like they tighten again.

What could be the cause? Is it just a chain slipping or is there something wrong with the bottom bracket?

Thanks!