2) insider with a bomb due to poor security. Egyptian authorities would be keen to play this down. But from the pax list this seems unlikely, as they were all Russian, unless something was planted in the cargo, so it can't be completey ruled out.
It is more likely if you know the current situation in Chechnya. Even with only russian citicens on Board. Chechens as I know are Muslims too.
A hydraulic failure is my best bet for the main reason at the moment, as the Airbus Rules dont let you do maneuvers on such a high G-Load that the Airframe would break apart.