If I leave mine parked on any suspension height with the timer relay pulled, it will eventually end up straining against the end stops of the shocks, way higher that "oh shit I'm beached" mode.
I think the non-return valves are leaking allowing tank pressure to blow past the "up" valve into the bag valve gallery, where it then blows past the bag valves.
Bear in mind that the valves are just seated by the spring and held closed by air pressure on the back of them - if there is sufficient pressure on the other side of the valve it will lift the rubber disc off its seat and the valve will leak. This is what the NRVs on the "up" and "down" valves are supposed to prevent.
I suspect this is why you get the "dancing front" problem when you're stopped at lights - with your foot off the brake the front suspension valves both open to equalise the pressure between left and right, and if the "up" NRV is leaking then air will just flow straight into the bags.
R-reg 4.6 HSE on LPG
R-reg 4.0 SE on LPG
F-reg Citroen CX 22TRS that has now been sold :-/