Do you see any eggs anywhere? Maybe the tank conditions are optimum for mating.

My first thought is that the male is "practicing" or getting into the mating stage. Young male frogs are known to prepare or "practice" for the mating season by sitting on top of females. I had a young male Wood frog who sat on top of my two female Wood frogs at the same time! However, he didn't try to mate with the females, maybe because the conditions weren't "right".