The Wall Street Journal reported that a flight attendant accidentally hit a switch on the pilot's seat, which pushed the pilot into the controls, forcing down the plane's nose
So either the switch is in a bad position or is too easy to hit.
During the incident people were thrown against the roof of the plane, which was travelling from Australia to New Zealand
Keep your seatbelts on at all times! Unless you're getting up to use the restroom or grab a bag, belts on. This is why they mandate this on most flights. Unexpected turbulence can do something similar.
Once had a flight attendant crouch next to my seat for to such turbulence. They were unfazed meaning this isn't uncommon.