What I've found, (if you can't find any other reason for this uncoupling) is any abrupt stopping of hydraulic flow, as in full up flow to abrupt "stop" or full down flow to abrupt "stop" will cause this on occasion but is not predictable or readily repeatable.  Also, trying to bounce the loader as if shaking something out of the bucket can cause this. 

Are the scv levers in the cab easy to move from neutral to up or down or do they have a 'click' style detent in the handle?  Sometimes this can cause a problem.  Only remedy I found is to use the levers in as soft a manner as possible, and try to keep from abruptly changing the flow of the hydraulics.

If this happens to describe your problem, then a joystick without 'sticky' dentents would go quite a ways toward solving this.

hope this helps