Findagrave members can delete memorials they created for any reason. It has been known for a member to have a dispute with admin and then go off in a huff after deleting all their memorials. Were the memorials you were looking for all in the same cemetery or in the same area, so they might have been created by the same person?
If the memorials were accurately created in the correct cemetery, does the Ancestry link give enough information for you to recreate them? Are there burial registers online that you could check to see if the memorials were correctly placed? I am wondering if they were deleted because they were in the wrong cemetery, though in that case they should have been moved to the correct cemetery.
If you are a member of the findagrave forum you could ask there, though I doubt if anyone knows the answer except the person who deleted them. Even if someone was upset with findagrave, I can't imagine why they would delete their memorials - all that work down the drain, it would be cutting off your nose to spite your face really because it wouldn't hurt findagrave that much.
