#1. To make a statement to WWE
If you saw Jon Moxley's post-Double or Nothing promo, it's clear as to how fired up he is and just how motivated he is. You can watch it for yourself below.
Check out which wrestler EC3 finds cute RIGHT HERE
He claims he's there for a paradigm shift and to give the industry a much-needed "facelift". It was clear that his "flipping off" after the show was also a message to WWE. He truly viewed it as a prison to him and only now are people truly realizing why he left and why staying was simply not an option for him.
Moxley is there to make a statement and AEW is just one part of it. By signing with NJPW as well, Jon Moxley is putting out a clear statement to WWE that not only does he not need them but there are plenty of others who are willing to give him the opportunities that he deserves.