You know I had this same problem before and it was due to the mouse. The button was sticking for a split second after I would click to close a browser page and like you, all of them would close. I bought a Logitech mouse after that and never had that problem again.
What brand is your mouse? It's not easily detectable that it's the mouse's fault. Try to borrow someone else's mouse and then connect it to your PC to make sure.
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