Mr. FiXiT

[Id_Jit]

A smarty-pants answer could say: “Because it’s drawing too much current.” <g>

More seriously Raw, that may not be so.
I pretty much agree with Dan and Phil as to the possible causes.
Include a bad starting switch, or timer; whatever is used in your compressor motor.

More info could help zero into the problem.
How long does it take for the breaker to trip?
Does the compressor start and run normally for a while, or does it labor heavily briefly and then trip the breaker?

Id

Edit P.S.
Hope you don't have aluminum house wiring?