As Datatek says, the GM Valve block was made from very soft materials....the constant modulation of the Valves inside there bores wears the bores away depositing the 'crap' into the box, this then acts like a cutting paste expediting the wear exponentially.
The Main culprit is the Torque Converter Lock Up Clutch modulation valve...as this wears, the hydraulic pressure to the LUC drops, so the system tries to compensate and modulates the valve more which in turn not only wears the valve bore more but also the LUC is put into action more than it was designed for causing premature wear to the LUC friction material...which gets dumped into the gearbox oil...again acting like a cutting paste wearing out the LUC material..the valves and the valve bores...and the cycle starts again....
Regular Oil and Filter changes can help prolong the life of the box, but it will eventually fail and oftentimes without warning too...!!!