Sorry, should've been more clear. The lockring on your hub (note the black lockring on top of cog) is loose and when you apply backwards pressure to your pedals, it loosens your cog and because your lockring is loosen, your cog could spin freely under the lockring. That could be enough to strip your hub, but I don't think it it stripped. It could be, I am not sure.

You can't really tighten your lockring with out a lockring spanner, they probably cost about $30-40 at your local shop, it's a good tool to have.

In the meantime, do not bust sick skidz around because you will strip your hub even further.