Revenge is very strong motivator.
All of Zlatan, Pogba and Raiola has a point to prove by being part of Mourinho's project.
Zlatan has never been fully appreciated in England. Pundits, media and many supporters doubt his ability to succeed in the PL. This is his last chance, his last mission. A challange like this is something a true warrior like Ibra never will resist. Succeed with United and his carrier is completed.
Paul Pogba probably don't have any bad feeling against Sir Alex. But the threatment he got from he old man can still hurt for a young ambitious man. Now he have a great opportunity to came back as the golden boy, the savior, in a new project together with a new manager. That could be, off course, together with tons of money, motivator enough to leave Juve. Maybe being recognized as a world class player by United (and Sir Alex) restore a little bit of his his pride.
Sir Alex called Mino Raiola the worst agent he ever have dealed with. Even if you're successful that line hurts. Being labeled with so much bad words by the Godfather of football probably have a huge impact on your ego. After all even Raiola is human being and his job is to look after his clients. Suddenly he's part of two, maybe three, mega transfers where his clients join United. That's means a lot, from his perspective it's a huge revenge.
I have read tho book about Zlatan and both him and Mino are similar personalities. They both had to fight to be something. They both think highly of themselves, have huge egos but deep deep down inside them you can see two insecure characters who tries to survive in a cruel world. Zlatan talks with pride about his father, his background, his shortcomings and his family. Mino Raiola does the same. They are both two simple men (in positive way) who tries to do the best of their talents.
I can be totally wrong but I think pride, revenge and hunger to prove your doubters wrong is a huge factor when you take big decisions. That's my take on these things.