Your conscious brain wakes before the motor centers of your brain wake. And no its not because of anything you do. Its natural. It happens to everyone, its just normally the thought/sensory areas and motor areas wake at the same time so no one notices the momentary paralysis. Usually if the thought/sensory areas wake up first its because of a disturbing dream or something in the environment that woke a person fast and the motor centers have to take a couple of seconds to catch up.
put an alarm clock and it will wake you up faster. Most sleep paralysis are fear induced since we panic when we discover that we are paralysed. Be strong and don't panic. Pray to your angel they will help you greatly even if you don't believe in them.
im saying this because im not the praying type and i did it once and it worked marvelously in a dream.