What is another example of something that starts moving suddenly?

Explanation:

Depends on your definition of ‘suddenly’. You could mean ‘unexpectedly’ or you could mean ‘instantaneously’. There are many things that move instantaneously - from the atomic level on up - depending upon how small a time interval you consider ‘instant’. And something moving ‘unexpectedly’ depends upon each persons expectations. Consider a car left in neutral with the brake off. If I know those conditions - its movement is not unexpected and therefore not ‘sudden’. If I don’t know those conditions and usual expectations are contrary - then it’s movement is unexpected.

And, of course, on the bigger scale everything is in motion in the universe. Therefore nothing can start moving suddenly. It may change direction relative to other things around it which we may interpret as moving ‘suddenly’.