I don't know what it is about the bed, but we had/have the same problem. Our 1yr old also only uses his bed, unfortunately we haven't been able to get him to stop. If we reprimand him with "No" he lays down on it, waits a little while, and then starts up again. Generally it's when he gets really excited like if you start jumping around or rough housing he'll run over to his bed.
I've been told this is a learned behavior, meaning that nothing hormonal or genetic is the cause. So when we got our dog he wasn't neutered and he humped, well now that he is neutered he still humps because he had already learned to do it.
The good news is that it's not often and he doesn't hump people or other dogs. The bad news is I don't really have a good solution, we just take his bed away and continue to reprimand him.