Many were surprised when he left. He never publicly stated but the talk was it was a family move.
There is not a specific NCAA rule against serving as both but the jobs are so vastly different and time consuming that D1 athletic departments would crumble with a part time AD. In the old days, they would name football coaches ADs to get them bigger paychecks and prestige. And the coach then would report to the University President directly. In those situations there was an Associate AD actually doing the job. I believe the last coach to have an AD title was back in 2008 at Louisiana Tech. Coaching is a 12 month commitment, especially with limited assistants like in soccer. They aren’t just figure heads. They are on the road (and overseas) in the offseason and much of in season.
The AD has to work with all the teams. The job function is much more desk work - compliance, budgets, logistics, branding, marketing. The hiring/firing part is actually very, very limited. Most ADs only end up hiring a handful of head coaches in their tenure as assistants are typically hired by the head coaches. And even during coaching searches, the AD is one part of it. There are search committees, interview committees, senior administrators, Deans, and the College President. An AD doesn’t have the autonomous power most think they do.