Lost reverse, need help finding it. 727

Need some help.

I don't want to just put clutches in and have this happen again.
Bigblock 727. Rebuilt years ago,red alto clutches, kolene steels. all new bushings, thrust washers, both bands, cheeta manual reverse valve body, not a lot of time on it, 50 miles at most, and a few 1/4 mile passes.
Just driving in driveway and lost reverse. The servo cracked. Replaced with billit servo. Pan was clean, no clutch material. New fluid. Car sat and the pan gasket was leaking so I pulled it and found what is pictured.
Pulled trans and found one clutch plate in the front pack disintegrated, the one all the way at the bottom.

Also found the fiber washer between input and output shaft was only half there.
And the edge of the front most planetary gear the edge was hitting and wearing the metal, making metal fragment on the edge.

What would cause just the one clutch to lose the clutch material?
And would the one input to output fiber washer cause the interference and wearing of the edge of the planetary gear ?

Thanks for the help.