Some may be ND's from not keeping the finger out of the trigger guard, some are clothing, drawstrings and their adjuster/keepers, or other objects getting snagged on the reholster. Clothing also can get into the holster as the gun is placed in it, then the person pulls the clothing free, sometimes resulting in the gun coming back out to fall on the ground, or the clothing snagging the trigger, followed by a loud noise.
Classes are an artificial environment. They are doing something multiple times, far more than in daily life. If one isnt watching the gun as it goes in the holster, the possibility exists that something could get snagged, unless you are wearing clothes that cant get caught. It would seem only form fitting clothes would meet that criteria.
Its great that people can carry for ages and nothing happen. That doesnt mean nothing can ever happen, and it doesnt mean youre a bonehead or dont know what youre doing if it does, just that things happen we cant always see or anticipate unless scrupulously careful with every single reholster. If you arent watching every reholster, it may happen to you one day.
Some people hold their thumb on the hammer as they reholster, which can alleviate the AD/ND. Others hold the safety on positively as they reholster. Some guns are more difficult to take preventative action with.