If the world has the same name as another world you have, it can spawn things from the other world into the new one. It has something to do with the way minecraft recognizes and loads files and saved chunks but I'm not sure of the technical details. Just give all your worlds different names and it shouldn't happen. It's just a glitch, nothing to get freaked out about.
The thing is, cows are very easily frightened. Bold, oversized and colored letters scare cows almost as much as all caps do. That's what's making them hide.
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