Cell phone cracked laptop screen?

Earlier today my sister left her Galaxy S3 on her Dell inspiron and closed the screen a bit. When she lifted the screen it was cracked on the inside ( meaning you can't feel the crack but you can see it clearly) We're still not sure what happened. Did the phone crack the screen. She didn't press down on the screen when the phone was in it. She told me she has done this many times before and this has never happened. Also the phone is perfectly fine.