I had a brown trouser moment with our grab wagon a few weeks ago; The boom moved on it’s own and clouted the control panel (scow ended headboard). I’d just had trouble with a couple of levers sticking and on checking for a fault, the boom shifted. It seems the rod/cable setup may’ve had a stone or some muck stuck in the connections somewhere. Certainly never heard of a hiab raising in transit though.
Our grab was refurbished and the controls are almost lightning quick, even on the drivers’ side.
It is possible for a faulty ram to extend by itself. If you retract the ram till it bottoms out, on one side of the piston you have max pressure, on the other you have no pressure, a leaking piston seal will allow some oil to travel from the high pressure side to the no pressure side until the pressure is equal on both sides. Any oil that goes to the other side will push the ram out. As you can’t compress a fluid, you’d have only a tiny amount of movement to equalise the pressures, but if you’ve got some air in there, then it’ll act as a spring and more movement will be needed to equalise the pressures. It’ll move until the pressure is equal to the load on the ram
We had this on a lifting deck livestock trailer, lower the deck when empty and a few seconds later it would start to lift up. The ram was coming out 2 inches or so.