Ok new theory (based on the screenshot):
The game calculates all the damage that will be going out (both from explosion and from the self destruct). But doesn't apply it yet.
The game resolves the explosion of gems, dealing the damage to the first troop form skulls. But because it has also calculated the damage it will receive from the self destruct, it factors that in as well, and decides the troop is deal at the point the skull damage hits it.
At this point, the damage going out has been calculated to all troops, but only resolved on the first troop. So Night Terror is still on full life.
The game then applies the damage from the self destruct and then kills the bombot. The damage kills the Night Terror, but because the game doesn't check for death until it's done the whole effect, the Night Terror's life drain triggers, bringing it back to life.
The game calculates ALL damage at the start, but only applies it at the point at which the troop takes the damage. Because the spell is in two parts, the Spider Queen takes (all the) damage and dies before the second part of the spell resolves and the Bombot dies, and therefor not triggering Spider Queen's Life Drain.
EDIT: A way to test this would be to have a case where both the first and second troop die to skull damage (if that's possible... it might be a case of all skull damage goes to the first troop regardless...) and then a third troop die to the explosion, all with life drain, to see which survive and which don't.