There are 2 ways that this happens:
1. If a vehicle rushes the gate to get out while the gate is busy closing , the motor takes a bit of time to react before it opens again, and this can cause the gate to hit a vehicle.

2. If the gate is set up to close immediately after a vehicle passes through and
someone is truck and trailing this can happen as well as when there is a trailer on
the vehicle because the beams see a gap and will start closing.