This is all pure speculation:
Mold usually takes a long time to grow on beer so as long as you're not leaving it in the fermenter for too long, you shouldn't have this issue.
Based on your cleaning , sanitation, and other handling techniques, I think you crossed that line. Someone else’s timeline could be longer or shorter based on their techniques.
While you did have a lot of headspace and the mold may have been in the carboy to begin with, my vote is you held it in the carboy too long.
My guess is your airlock is attached to your carboy with a stopper. That stopper allowed CO2 out and air in (based on the law of partial pressure) over the lengthy time you were holding it in the fermenter. That air that entered brought hitchhikers in the form of mold.
Sent from my iPad using Tapatalk