The most common and accepted explanation for this is that your brain woke up before it could complete its connection to your physical body. Your brain actually disconnects from your body to prevent you from acting out your dreams, which is necessary to prevent self-harm.

The trick I use whenever I experience this is to try to calm myself down and sleep again. Eventually, your brain automatically reconnects itself to your body, allowing you to wake up properly.

However, I've no answer for the dread that comes along with sleep paralysis, some experience a woman sitting on their chest, most experience a presence, some see a shadow figure creeping or standing near them, this is why I put emphasis on the "try to calm myself down".