Wiz and Amber Booty Call Triggered Divorce But it Wasn't Cheating

Wiz Khalifa left Amber Rose weeks before she filed for divorce ... and the trigger that sent Amber's lawyer running to the courthouse was an uninvited 2 AM  house call by Amber ... 

Sources connected with Wiz tell TMZ ... the rapper is telling friends there's a simple reason why he left ... "I just didn't want to be married to her anymore."

We're told Wiz moved out and got his own place, and the day before Amber filed, she went to his house at 2 AM and found him with another woman ... and went nuts.

As far as Wiz is concerned ... he wasn't cheating. He told friends he made it clear weeks earlier the marriage was over.

It's interesting ... even though Wiz moved out weeks before ... he listed the legal date of separation on his divorce response as Sept. 22 -- last Monday.

We're told Wiz is also convinced ...  Amber was secretly banging Nick Cannon -- something they both deny.