Seems most likely there are two possible causes:
1) midair breakup due to a mechanical fault. Who knows? It's all speculation here. We have to wait till the black box reports are done. No one seems to know why the captain was reputedly concerned about the plane condition.
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.
However this is just speculation again. This thread is just full of it.