Time and the crash investigation team will reveal why this aircraft came down, not speculation.
If this was an Isis planted device that brought the aircraft down then why are they keeping so quite about it?
Perhaps it could have been a group or individual who are sympathetic to ISIS operating without the backing of the leaders of ISIS who are now perhaps a little bit concerned that the wrath of Putin and the Russians will come down on them.
ISIS describe themselves as fighting a war, they may have just got their wish.