Electrical Question - popping a breaker

[Footloose33]

I have four ganged ABC switches and they connect to an in-engine room breaker panel of sorts...

Switch one goes to stbd engine starter
Switch two is a transfer switch
Switch three goes to port motor
Switch four to genny

I usually keep the engines on their own batteries by turning the transfer switch to off.

Sometimes - not every time - when I go to start the port motor the breaker pops. Zup with that? What should I be looking for?

Thanks as always