I would guess that the hydraulic oil leaked forward into the engine clutch compartment(dry clutch) and that made the engine clutch slip. The rear clutch runs in oil (the wet clutch) so hydraulic oil would not affect its operation. Check for leaking seals that allow oil to go forward into the dry clutch compartment. If the shifter collar that was broken was in the wet clutch asssembly, and it was dry, then you would have more leaky seals to replace. Anything can happen, maybe all that oil is going forward through bad seals.