There were probably many reasons why he joined NiP.
Just some from the top of my head:
Wanted to stand on his own legs and not only be "the apwer in Astralis".
Social/family situation, he already lived in Stockholm, has a Swedish girlfriend and so on which makes his job closer to home.
Possibly some chemistry stuff with other players in Astralis.
I don't think he or anyone else expects that NiP will be as successful as Astralis.
It's a new job closer to home where he can try something new and just be himself.
It's more than just about winning, it's about having fun with people you like.