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Before you changed the file back did you see the last time it was modified? might give you a clue what happened.  As Alan said check other files have not been changed (malware/hacked)  again checking the modified dates gives you a clue ( files you know have not been touched having a recent modified date, all the files having the same modified date... etc)

In answer to question 1 - yeah. Happens all the time. They're spammy sites that are scraping the serps to get some relevant content. There is nothing that you can do about it and personally I wouldn't worry about them too much.