calfskin flesh hoop shrinkage, and glue drying out and losing its adhesion, and temp changes over time of the wood swelling and then shrinking.

I don't think tuning has anything to do with it. Temp and humidity issues over time and dis-similar woods are most often the culprit IMHO.

Fascinating, I always back off from tightening tension rods though, I feel like something might just break or something if I overdo it.

Wood has a memory. Whether newly cut, or old vintage, it always will seek to return to its original shape, and will also react to any and all changes in temperature and humidity. This accounts for why so many older vintage drum shells show reinforcement ring separations and other shell irregularities. I have never seen well made and well cared for drum shells suffer problems as a result of head tension or just being played. Here I must again stress the words well cared for as the important determinant, for misuse and abuse combine to add years of aging to circular barrels of wood which would otherwise be straight boards and planks if not for the heat and glue manufacturing processes holding them in place.

BUCKIE B-excellent post and you nailed it perfectly.

Wayne