See, thats exactly the point. I can also come up with random reasons WHY, but neither you nor I have any idea wtf is actually going on.
If you look at this from a dramaturgical point of view, the whole point of the rendered cinematic was not that Tyrande and Sylvanas were fighting and that Sylvanas escaped. The important part was HOW Sylvanas escaped and that the NW powers failed.
Why that happened is the most important aspect of the whole cinematic and partly also of the following main questline. Leaving it unanswered is really a problem and no matter with what ideas we come up: Its fanfiction and nobody cares about fanfiction.