We've seen this happen from time to time with these diamond disks. We've continued, over the years, to refine the attachment structure of the velcro backed diamond disk. The mandrels now have a much better hook system than in previous generations, so you should see less and less de-tachment occurring.
One of the main reasons for the disk to come off the mandrel is fatigue and hook damage. The hook part of the velcro is the most important part of the equation. They hold onto the loops from the disk and prevent any movement of the disk. Eventually, hooks do wear down and loose their "gripping" power. This occurs mainly from normal usage of removing and applying disks over and over.
It's entropy and there's not much to be done about it.
There are things that also speed up this fatigue on the hook system including heat. It's very important to use the disks with adequate water so you don't weaken the hook system by building up too much head and stunting the tips of the hooks on the mandrel. This will seriously weaken the attachment of the loops to the mandrel.
There are some usage environments that are tough on the these system too such as edge work or areas that constantly use only the edge of the disk and not the full surface. This puts undue stress on the attachment and can cause premature failure of the system. Once again, there's not much that can be done in these situations. If that's how you need to utilize the disks, then you'll have to get comfortable with more disk attachment failures or look into using our Dual Loc or Roloc attachment systems.