I can't tell you exactly what you're experiencing, but I do know that there is a "feature" in jemplode whose stated purpose is to do something like what you're describing. I can't even tell you exactly what it's supposed to do, except that it seems to automatically propagate changes to a playlist's artist field to all of the items underneath. It doesn't seem useful to me, and even if it's useful to others, I, too, found out about it the hard way.

I wish to echo the sentiment that whatever this feature is be optional if it isn't already, and certainly defaulted to off.