So a possibility I've seen brought up is that the travel between world's will, in-universe, be us going to one of the shards. So my theory is, we are not causing a Flood of Ljght, but the war with Garlemald is on the path to Calamity...and if the Calamity happens, Hydaelyn won't be able to stop the Flood in the first shard.

It could even be that the other Scions have already been pulled over to try and speed up the process of stopping the Flood, while the WoL/D will go between the Source and First Shard to try and prevent both.