I am saying it is this thin shell design and the combination of Memrilock hardware. They are however, very durable under normal conditions. What you are describing for use isnt exactly normal. There is some inherant fragility to thin shells. With any thin shelled drums.
When new, these were rock solid. Youve got 35 years of age on these drums. And with that, unknown abuse in handling over the years. My set was purchased new in 1977, I purchased them from the original owner in 1979. They were rock solid until the day someone moved my drums with the two toms still mounted and cracked the shell at one of the spur mounts. I reinforced the shell and got another five years of use out of the bass drum. When I changed my set up to only mount one tom on the bass drum.... it happened to be the left side was the weak side, and I found a replacement BD on ebay in the same color from the same serial range. I replaced my BD, reinforcing the spur mounts using the afore mentioned fender washer reinforcement. That was done in 2000, and I have not experienced any further problems.
When you raise the front of the shell a couple inches off the ground, you have to lengthen the spurs accordingly. This puts them further from the shell, increasing the amount of stress. Granted you are not putting the weight of toms on the bass, which would significantly increase that stress on the five ply.... THIN .... shell. But the elevation, lack of weight, and as you stated... "hard playing" are causing your shell to actually bounce, and therefore, creep away from you. The creeping problem could be alleviated somewhat by tightening the clamp of your pedal to the BD hoop...... providing of course your pedal is one of more modern design that does not "BITE" into the hoop. I have used a Tama Iron Cobra since 1999 and I do tighten the clamp to the hoop, without any adverse condition issues to my hoop. I am using 2" twelve ply natural maple hoops on this drum, and have had them on since 2000. If tightening the clamp would warp the hoop, this would have already been an issue for me. These hoops are significantly wider than the originals.
Rogers XP8 series of drums were introduced in 1979, eight ply, all american maple shells. These drum shells are a bit thicker, and stronger, and, handle the stresses of the memrilock hardware much better.
Currently I have two sets in the five ply, and two sets in the XP8 shells.