We've no idea what goes on behind the scenes. Could be a whole host of reasons why hes moving on- personality clash, work ethic, couldn't agree terms on a new deal or maybe just maybe Dougie has identified someone he believes is better at the job.
Agree with SpoonTon as well, he worked with good keepers, can he really claim he made them better, I'm not so sure. Jack Hamilton had been in Scotland squads, Schwake had been out on loan and impressed at Annan and was highly thought of at Livvy and Mullen looked the part almost instantly. I'm not sure we know for sure he's been fantastic and successful etc, he may well have, but we only see him doing his work for 20-30 mins on a Sat.
We need to trust that the manager knows what he's doing, he works with his staff and players every day and knows their strengths, weaknesses and personalities a hell of a lot more than us.
Good luck to the guy, hope he find something.