(edit) Brushless Motor - Why does the stator burn up if the rotor doesn't rotate?
This answer was given prior to the op changing the question. Originally, the question referred to the motor as being a squirrel cage type. Subsequently, after several comments, the op changed it to a brushless motor. You just can’t get the staff any more!

When you stall a 3 phase motor it becomes a transformer and, because the rotor is the secondary AND is shorted out (by necessity for it to operate as a motor), you end up with a shorted transformer.

Ask yourself why a transformer burns when the secondary is shorted.