Edit: Disregard the "fan fiction" theory below. It's bad. I'm bad. I should read more on the lore. Didn't realize Alleria was taking blood elves that were already banished for using void magic. That throws my theory completely out of the water. I also realize it was out of character for Lor'themar, but I was also assuming some old gods might be involved, whispering into his ear.
I have seen several theories as to who or what burns down Teldrassil, but I've yet to see anyone really discuss Lor'themar as a possible culprit. Apologies if this has already been brought up, but out of everyone in the warcraft universe it seems to me that Lor'themar might have the greatest motive behind burning Teldrassil. Alleria, as we know, will be taking in her fellow brother and sister blood elves and high elves and training them to become Void Elves. In doing so she will cause a rift in the Blood Elf society leading many to essentially defect to the Alliance.
I am of the opinion that this act will anger the ruling class of Blood Elves who will in retaliation send a massive fleet to Teldrassil and burn it to the ground. Sylvanas, who is busy devising her own plan to attack the Alliance city of Stormwind, will be caught off guard by this act of war and will rush to stop Lor'themar only to arrive too late. This attack may also lead to the death of her sister Vereesa who rushed to the aid of her people only to fall in battle. Sylvanas knows now that the spark of war has been ignited...literally...and she will have no choice but to move forward and begin preparations for full scale warfare. Knowing a retaliation will commence soon she ports back home to the Undercity.
Now, this could also explain the absence of Blood Elves in the assault on the Undercity cinematic. Either Sylvanas is pissed as hell with Lor'themar and has put him in time out, or more likely he is mounting his forces in Hyjal and continuing his campaign against the Night Elves. This also falls inline with each continent becoming dominated by their respective factions (EK=Alliance, Kalimdor=Horde). Lets also not forget that these events will do nothing but fuel the old gods and some of our leaders on both Horde and Alliance may be influenced into doing things that are irrational and impulsive. A divided world will be much, much easier to conquer.
Anyways, that's my two cents. Let me know what you think.