I've been having the same problem and it just happened twice for me in the the last three minutes. I do not like having my screen hijacked. Makes me very nervous about viruses.
This has been happening on my iPad for weeks. It is very annoying especially when I am in the middle of a thread. I have to close it out and find the thread again. Last night I did this and got the pop up on the new windows I opened to try to get away from it.