In this age of technology that tries to predict and guess our intentions based on patterns and hand wavy algorithms, it's also safe to assume that autocorrect just outright got it wrong when it corrected an entire phrase. Much as grammatical faux pas irritate my mild obsessiveness, I've learned to just let them slide. Personally, I try not to let such edits slip past me, but they often do. It's usually IM and the person on the other end gets a good laugh, but the intent is typically evident.