Hi, I only fly with the keyboard, but there could be a couple of things at play here.
One, uneven terrain. Arma has a very slippery terrain/tracktion model for helos on uneven ground, and a slight hill will sometimes see the chopper "sliding" off down the hill. Can't recall the times we've landed and moved away from the chopper, only to look back and see it sliding away down an embankment to it's doom, lol.
The other thing could be with the control layout. When I land with keyboard controls, I can sometimes get a better "stick" to the landing if I hold the Z key (for throttle down). This will sometimes seem to make the chopper stop and stay stopped while held down. You can manually try that and see if it helps, and if so try to find a way to work that into your joystick layout... Not sure if that's possible but could be!