How the Order broke up is never mentioned in-game, or it has but I didn't know. In this blog I'm going to share with you my theory on how the Order broke up.
Firstly, the Order lied to the world. This made Ivor hold a grudge against the Order, and eventually left. This caused a huge impact to the Order, and the start of the legendary group slowly disintergrating.
Then Magnus did something to Ellegaard that made her extremely upset, or vice versa, and caused their rivalry, then they both left.
With only two members of the Order left, this drove Soren crazy. He is sick of humans and them not treating him right, and departs to the End to test out Enderman.
Gabriel then goes off to do his own thing.
Anyway, I hope you liked my prediction. Leave your thoughts in the comments.