Why would a gelding try to mount a mare?

There are several reasons for this. Most likely, the gelding was gelded late. This means that he would have the characteristics of a stallion, and possibly the mannerisms (mounting etc.). This could occur if the gelding has been gelded a few days ago, and not a few weeks. It takes about 3 weeks for the testosterone to flush out of his system after he's been castrated. Another possibility is that when he was castrated, the person doing the castrating didn't take out the spermatic sac that is above the testes, which would cause the gelding to act more like a stag or a stallion than a real gelding.