I was frustrated by the seemingly inconsistent length of time my toon took to be cleared from combat (i.e. the amount of time before I can mount or interact with an object once all mobs are dead). With some experimentation, I determined that the game does not clear a player from combat until all internal spell timers have cleared. For example, if I use the Dragonknight Searing Strike ability, I will not be cleared from combat until all instances of the 8.5 second d.o.t. have cleared (even if the mob expires, I must wait for the allotted time to pass). If I make the same kill using only Lava Whip, I am cleared from combat the instant the mob expires (no d.o.t. = no wait).
Why does this matter? It is very frustrating to kill a mob and have another player take the node/chest because you are stuck in combat or to be left behind because your group has mounted and left for the next objective while you wait for the in-combat flag to clear.
DEVELOPERS: Please have all players clear from combat the instant (or at least a set amount of time after) all engaged enemies have been downed.