The reason it does that is b/c it goes off just your regular rear brake, which every time you use it is wears itself down a little bit, so over time it loosens a lot. It is a known issue on any rear disc fourwheeler, and all you need to do is tighten or, or do as crash says and leave it in gear, which is what I do. I am going to block it off tho, as i never use it, so I don't have to worry about it.
2008 700xx; Power Bomb header with power core 4 exhaust, Power Programmer III, pro armor nerfs, prm products control front and rear bumpers and full skid, 10" ITP rears, ITP Baja's fronts, RZR 2's all the way around, Uni filter, 13t sprocket, CCP steering Damper, DS450 shock swap, Dual Gasser Throttle, bar risers and Moose barkBusters. 730XX kit plus other engine mods!