Every motor when it’s spinning generates voltage, even if the motor is spinning under its own power. It’s called back EMF. That’s actually what the KV rating of a brushless motor really is. A 2,100KV motor makes 2.1 volts of back EMF at 1,000 rpm, although our common usage of 1,000 rpm at 2.1 volts works too.

The motor generates that same voltage when it’s spun mechanically too. In this case, it made enough to light up an LED. Every now and then you can see guys moving a surface really quickly and the other servos jump. It doesn’t hurt anything.

Not every motor -
One must be of specific design ...... there are many