Couldn't it be new stress? Turning, temp and humidity changes? New stress and not "retained" stress?
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Not likely.
The stress is retained in the wood and as the outside layers, which are holding the stress in check, are peeled away,
any internal stress now has greater freedom to cause the wood to move. The stress is relieving itself.
Take Gary's example. It was straight when he got it, after hanging 15 yrs and after a few cuts, it decides to move.
Had he not cut it, it would have stayed straight. He peeled away the outside layers, the stress retainers.
This is a strong arguement for taking small cuts over an extended period of time.
The wood may very well move during this process but that doesn't mean it's trash.
The closer you get to usable diameter and it's still moving, time to start thinking trash.
Wood movement doesn't happen quickly/immediately. It takes time to show itself.
The longer you can let it hang, the better. 'Cue-maker's time' is very valid for this reason.
You can rush the build but you may not like the results.
KJ