Can someone tell me what causes eyes to get proptosis?

Lots of things. There are a variety of ways to think about this but the easiest is from an anatomical view. Basically anything that pushes the eye forward in the orbit can cause proptosis. This could include infection, mass/tumor, bleeding, enlarged eye muscles, trauma to surrounding tissue, inflammation, etc. Also certain eyelid problems could cause the eye to appear more proptotic.