Probably because it was seen as somebody editing a post that wasn't theirs. In this case it's not neccessarily obvious that the original poster didn't fully register and so probably didn't get the same session when they came back.
I'll edit it back in.
To the comments: The pros and cons of the unregistered editing and accounts system is really quite irrelevant here. It is what it is.
I considered the edit to be probably from the OP. I made that assumption on the basis that there's nothing harmful in the edit (it's not spam) and it's a continuation of the question, adding more information, and that it doesn't conflict with the existing question (unlikely to be a hijack of some sort).
Oh and that doing that "maliciously" accomplishes nothing. And even if they did, the OP could roll it back, flag, comment, etc to let us know that something untoward had happened.
I'd expect people to treat these sorts of edits with scrutiny but not absolute and automatic rejection.