I think it's because as the rider is leaning forward to indicate a desire for forward motion, the board drives the motor to move you forward, but also counters the natural tendency of the front lip to just touch the floor.
If the motor cuts off while you are leaning forward, the lip will immediately hid the floor and stop you. And your muscle memory isn't expecting that kind of balancing, having been used to the board "fighting you back" on leaning forward too much.