First thought, breaker is undersized for the irrigation pump's electric motor.
Could be a number of problems. Line voltage not up to standard. On 220/440 volt, could also be that one electrical wire supplying electricity to the pump is not equal to the other one/two.
Amp probe to each wire supplying voltage to the pump's motor. Record each wire's amperage during motor start up and then running. Also check each wire's voltage.
Try the following: motor's rated horsepower times 750 watts = ??? watts. ??? watts divided by line voltage = amperage.
Example: 10 HP times 750 = 7500 watts. 7500 watts divided by 220 = 34.09 amps. In this example the amperage would top out at close to 34 or 35 amps on motor start up.