Circuit breakers are engineered to deal with the brief peak starting currents for devices whose running current falls within the specified amperage. A typical breaker on a 120V circuit will tolerate a brief starting current of 250% of it's rated current, so a 15A breaker will carry 37.5A during the milliseconds before the rotor starts turning. A 1-1/2HP motor that draws 12-14A at "full load" would be expected to draw 25-30A during startup. It should not be surprising that this popped the CT while the branch circuit breaker remained engaged.