The reason that most people wake up suddenly from deep sleep is stress and thoughts racing in your mind which can also sometimes keep you awake and prevent some people from sleeping.
Waking up suddenly from deep sleep can also be caused by a nightmare or dream that is affecting your deep sleep.
Stress is the most common reason that makes some people wake up from deep sleep and trying to avoid that stress can help stop the sudden waking up.
Sleep disorders or even depression can cause some people to wake up suddenly from a deep sleep.
If you continue to wake up from a deep sleep suddenly then you should speak with your doctor because you might have a sleep disorder that needs treatment to help you stay asleep.
However suddenly waking up from a deep sleep is not harmful and as long as you can get back to sleep at night after waking up suddenly you should be fine.