Mauro Ranallo was first contacted by Michael Cole via email to see if he was interested in joining WWE in September. Ranallo then signed his WWE contract in October. Triple H was the one to finalize the deal with Ranallo to bring him as the new lead announcer of SmackDown in 2016.
The belief was that key people in WWE had seen him do the Mayweather-Pacquiao fight and also knew he had a pro wrestling background, so it made sense to contact him.
WWE made it clear that Ranallo would be allowed to continue announcing boxing, kickboxing and MMA while outside of WWE. The plan is for him to work all WWE PPVs and SmackDown TV tapings.