When shorting power leads to an unenergized bldc motor, it locks up why?
You are discovering why some of the free energy schemes don't work. Many of us, in our youth, have invented the idea of recharging an electric vehicle's batteries from a dynamo attached to the wheels. The problem, of course, is that the energy has to come from somewhere so drawing energy from the dynamo causes mechanical resistance on the dynamo shaft and this has to be overcome by the driving force.

In your case you have a short on the output of your "generator" so any motion causes a current to flow, limited only by the motor's internal resistance. Even though you are doing no useful work outside the generator the power being dissipated internally has to come from the generator shaft and this is the resistance you feel.