Internet Explorer showing error while opening many pages

I was writing an article in Word of about 3 pages and I also had to do a lot of research for it, so many pages of the Internet Explorer were open. When I saw how many of them I had opened I tried to close some of them and a window appeared with the following message. Can someone please tell me why did this happen?
End Program – steam.exe
This program is not responding.
To return to Window and check the status of the
program, click cancel.
If you choose to end the program immediately, you will lose
Any unsaved data. To end the program now, click End
Now.
End Now Cancel
