The people that went back to Ryanair seem to all have gone back for the same reasons:
a) the quick command upgrade promised at the interview at Norwegian has now been moved into the dim and distant future and instead of a 6 month wait, people are now looking at 1-2 years according to current plans.
b) the people that left Ryanair to get away from the dodgy contracting employment practices of setting up Irish Ltd companies, have now signed permanent contracts of employment directly with Ryanair on their return. This will mean no further unpaid leave, basic salary, sick pay, pension, etc etc etc.
c) returnees have been given their first choice of base mainly and been promised a fast track to command within 6 months. This, unlike Norwegian, is actually set in stone instead of an empty promise.