Who do we think blows up the tree?
Thinking SI:7 could do it to foil Sylvanas' hostage plans, but I don't think those characters are prominent enough to be a satisfying raid boss.
No one else in the Alliance seems to have the balls, maybe Maiev but I don't think she's ever been involved in faction warfare and just had somewhat of a redemption arc. Same thing with Moira and the Dark Irons. Hardly die hard pro Alliance characters.
Gallywix could do it for lumber profits, but going after Azerite was his idea so he'd be with Sylvanas' plan.
Blood Elves have form for doing this kind of thing and perhaps have a grudge against Night Elves for their exile. Not finished the Dark Elf story yet but they seem like assholes too.
Rest of the Horde wouldn't be doing this as it's against their war plan.
Seems unbelievable that Jainia would do it, but if it is Blood Elves she can go crazy and start slaughtering them again. She is going to be a central character to the expansion it seems because of that short.
Oh wait... Azshara has a short too... OK I wasted my time typing this up. It's her obviously :/