When I press the start or stop button in the MCB, and then when I power the MCB it is getting tripped, so what is the reason for that?

When I press the start or stop button in the MCB, and then when I power the MCB it is getting tripped, so what is the reason for that?

It could be because of two reasons
Because there is probably a short circuit (or) wiring issue in the control circuit (or) associated load, the MCB trips when you click the start/stop button and turn it on. It might also result from:
NO/NC connectors are not wired correctly.
Phase-to-phase or direct phase-to-neutral fault
Shorted motor (or) contactor coil
Overload condition
Check the load connections, insulation, and wiring.