In the past, the night elves abandoned arcane magic because they saw how dangerous it was. The use of Arcane magic was drawing the burning legion to Azeroth. they
Now, with the return of night elf mages, there is more arcane magic in the world. And without the focusing iris being in the nexus, which was the focal point of all arcane magic, Azeroth will be more of a beacon to the burning legion.
They have already been to Azeroth three times due largely to the use of arcane magics, and my speculation is that with night elves re-learning arcane magic, we'll become more of a beacon to the Burning Legion, thus setting us up for another encounter with them, and [hopefully] Sargeras.