I have an older MH with a single 14' slide. When I arrived at the MH today, I noticed that the slide had partially opened (and yes, I had the safeties on...it literally pulled one out of the floor). ...
It also depends on whether one end of your slide is creeping out or both. If both are extending and you have the HWH slide mechanism more than likely it is the retract solenoic as Dennis mentioned.
However, if only one end of your slide is creeping then it is more than likely the hydraulic ram on that end of the slide leaking internally, or externally if you see evidence of a leak or hose leaking.