marad,

You've ruled out windstorms but make no mention of lightning. If the barn
is old and in bad shape it's possible that the insulation on the wires is
also old and in bad shape. From your mention of trees and branches you
suggest that no one has been maintaining this service line. I don't think
that the breaker on a line transformer is intended to protect your service
lines. You should have the power company take a look, and replace or remove
this service line.

Good luck,
Dave M.