Mastery page opened on its own.

file:///C:/Users/pc/Desktop/nooo.png So I was getting ready to do a match, & while I was typing, my mastery page popped up. I attempted to close it, & then received a white box with nothing but the "x" in the top right, & the first line of chat from anyone who typed. I explained my situation, & hope they understood. I'm not sure how this could be repeated. I was just typing, & then boom. there it is.