Uub realized he was weak, seeming how Goku never turned Super Saiyan when they fought and then he learned from Vegeta that he is the reincarnation of the toughest enemy they ever fought. So Uub began to harness the power of his past self, but also his personallity, resulting in his second form. He then set sights on
Uub's Second Form
destroying new york but the tmnt, and gohan (who was there for colledge) teamed
up to defeat them. Gohan, realizing it was Uub tried to calm him down. He lost some power, but he regained control resulting in his 3rd form.