Are you fans coming on on the condenser?
Like Gary said, it could be a number of things, but for whatever reason the compressor clutch is under over than normal stress which is what is causing the breaker to pop. This could be because there is too much or too little freon charge, or a clog or blockage in the system. It could also be because the condenser is not being cooled, which is causing high pressures in the system, which in turn would make it difficult for the compressor to pump freon through the system.
"It's always wrong, everywhere, for anyone, to believe anything beyond insufficient evidence" -- William Kingdon Clifford
John (N9MXX) & Derek (KC9KEM)