I can think of four reasons, listed from most to least likely.
The breaker may be a GFCI breaker and your motor causes temporary imbalance on ground.
The breaker may be an arc fault interrupt breaker and switching the motor causes a temporary arc.
The motor may cause a temporary surge dumping its momentum and temporarily generating breaker overload.
And, the motor may have some winding/brushes that cause temporary shorts on shutdown.
It's most likely GFCI or AFCI, which is why people ask about whether the breaker has a test button. But note that running motors have momentum, and a motor is just a generator working in reverse, so when you turn off the power, the momentum needs to go somewhere, and if there's a party between one connected phase and ground (even with significant resistance) the motor may try to dump it's generates energy there.
For a bigger motor, like your 1/2 HP, the best option is to use a motor control relay or solid state motor controller, but that's not always practical or economical.