Current most popular theories among Egyptians:
1) Israel did it, using recent training exercises as a cover
2) ISIS did it, taking advantage of of weak French security and everyone should ban flights to France (this one is a little vindictive)
3) Turkey did it. We're not sure how, but it sure as eff just seems like something those bad person Ottomans would do.
4) The U.S. did it, because 1999 Egypt Air crash
5) A "western power" did it using new EMP weapons because that's just the sort of thing we believe around here.