"All of them" is definitely an overgeneralization for just two explanations. A decent percent could be unusual weather phenomena, secret balloon tests, etc., and some are just too weird for me to feel comfortable with any obvious explanation even if I don't believe they're likely to be supernatural or extraterrestrial. I'm sure there's a mundane explanation in most, if not all, cases, but sometimes that can be all but impossible to piece together later and involves something no one would think of at the time.