Even if we don't give a legitimate explanation beyond "Oh hey this is a thing now," somebody is going to have to answer the question of "ok but why" eventually. Heroes has always made at least a token effort at explaining every Banner concept(except Fallens if I remember right, that just kind of happened), generally using the same "uhhhhhhh they have a festival for that here" option, but there is no way anyone is going to believe Askr has a "Rule 63" festival, that's going to be a laughably bad explanation. Loki in Norse mythology has a history of swapping genders and this Loki would absolutely think up the idea of popping open a dimensional hole to a Rule 63 world, so you start your R63 seasonals with a Tempest event since that really works with the original function of the Tempest, have it be Loki's fault because she's literally said to Merth in a prior Tempest that she does these things because why the frick not, and there you go, you've fulfilled your basic FEH quota of pretending to explain why this is happening and only started happening now, even people who are like this is stupid have to acknowledge that at least they used the one source in the game that it could feasibly be explained by to attempt to explain it.