WHO KNOWS?? Your browser malfunctioned. Could be that you needed a restart. Could be because your PC is infected. Could just be because you ahve not updated. Could just be a freak accident. We can not tell you because we do not know what is running on your PC, what sites you go to, if you have been infected wtih anything, if your up to date among every other factor that goes into PC usage.
So until you go thru your system with a fine tooth comb and search and score it for every possilbe reason why this could have happened we can not tell you why it happened. We are not using your PC so we have no answer for you. You have to troubleshoot your own PC and find out what is wrong with it. We can not give you a answer for everyhting that could possibly be wrong with it cause we do not know what you do with it.