lonely shubunkin

We have some fish in the pond only because a friend moved and begged us
to take them. There a a few goldfish and one larger shubunkin. I think
it's a shubunkin. I can't find anything else that it might be.
Anyway the shubunkin is driving the other fish nuts. His favorite game
is to take the fat one (maybe some sort of oranda) and push him around
from the side or underneath. The little guy can barely swim as it is
with his short stubby fins. He will circle him around the pond 2-3
times before he gets away.

So, is this some sort of mating thing? The shubunkin doesn't bite or
pick at them, just loves to push them around. I find the other goldfish
hiding (and panting heavily) in the veggie filter. If the shubunkin
finds them he will root them out, or sometimes just wiggle in beside
them. That's what made me think it was a mating thing.

Mark b.