At the moment, there does not seem to be an explanation as to why this occurs.
My HTC TP2 has been using S2U2 for awhile and the volume controls would always lock when the screen locked. NO MORE!
One day they locked and the next they did not. I don't monkey around with installing all kinds of programs, so the device remained essentially the same from the time I installed S2U2.
The phone uses 6.5 but was never upgraded after installing S2U2 either.
Thus far, I have not found a setting or program that is causing the conflict with the device controls.
Over at XDA Developers forum, the same question has been asked, and no resolution found so far.
I sent a support email to the developer of S2U2 and am waiting on an answer which I will post if received.
What I did to limit the problems is to turn the phone in the case so the volume buttons face upwards (case is horizontal) not the bottom which reduces pressure on the keys.
I did not try a hard reset because at this stage, it is not worth it. The volume controls are NOT normally locked without having S2U2, so there is no real problem with the phone.
Wish I had a better answer.
