Yup. Like the rest of the Third World, tarmac security is a joke. Half the baggers are probably ISIS or Al-Qaeda.
Either timed or altimeter triggered
Also dead on. I vote altitude triggered: it appears to have happened as they reached ~ FL300. Simple pressure differential detonator, set to typical cruise altitude pressure difference.
Explains everything, and is simplest: it's no accident that something like this happens to a Russian airliner in that part of the world at this point in history.
No kidding! Tarmac security over there is scary!!
I watched a burka clad woman (?) board an aircraft with only slits in her headdress for eyes - no baggage check - and was horrified.
The Sinai is a mess...