Don't have much of an imagination, huh?
Why couldn't she have been invited by her sisters to their reunion there, only for some third faction to ambush them and burn the tree, putting the blame on Sylvanas?
Remember, back before Cata tensions were inflamed between the factions by Twilight cultists ambushing a meeting of druids in Ashenvale dressed as Horde soldier's. I see no reason similar couldn't happen here.
Sylvanas' stance in the art looks more like she just regained consciousness, stood up, saw the tree and is thinking "well, shit."
The point is Blizzard specifically didn't say she did it
Maybe she did - even most likely she did - but it's by no means confirmed. It could wind up being just about anyone else, and it wouldn't be contradictory to what we currently know.