Cat 303.5 Pump Coupling Issue. Nylon Flange

I wish to be enlightened by someone with experience on this issue that has presented itself to me. I have a coupling failure that I am having trouble rationalizing. First off there is a video on youtube of a guy having a similar issue but it appears that his pump side male coupler slid forward on the shaft and went through the plastic female gear tooth section, disengaging power transmission. In my case the male coupler did not loosen on shaft but did pass through female flange, only 80% though retaining 20% engagement until finally stripping both metal and plastic teeth also contacting flywheel bolts. It appears to have been in this position for a while. Here is what I don't understand. The male coupler is as far back on pump shaft as it can be so it wasn't simply installed too far forward. Did the pump shaft extend out of pump due to thrust area wear? Did crankshaft walk over into pump shaft? I could feel no axial play in pump shaft. My bush fix for now to get the machine out of th hole is to machine the back face of male coupler so I can slide it back farther onto shaft thus engaging the remaining 80% of tooth surface. I know there is something else going on here. Thanks. Dan Romano