https://youtu.be/4SD6iTKfhLo?t=1121
Sylvanas wished to capture Teldrassil to end the war before it even began by massacring all of leaders in one glorious day for the Horde to inflict a wound that would bleed the Alliance dry of its morale.
It would seem Sylvanas never intended to burn down the tree but capture it. Which means
A: She or someone else accidentally burned it down.
B: Someone of the Alliance wished it to be burned.
TLDR:
Sylvanas tried to avoid BfA by CAPTURING Teldrassil and killing Tyrande, Shandris and Malfurion.