When you lower the coach you initiate the opening of the three 3 port Norgrens that exhaust air as well as the shifting of the three 5 port Norgrens to open the path from air bags to the exhaust.

If I were to take a guess as to why your aux system is leak free except after you lower the coach I suspect the answer lies in the air circuit that relates to lowering the coach. Going further, unless you tell me otherwise, this only occurs when lowering the coach and the issue goes away when the coach is raised or asked to return to the driving height. That being the case I would focus my attention on the three 3 port valves in the steer compartment, but not rule out any of the 5 port Norgrens that might not be shifting properly, or more precisely not returning to the off or closed position due to weak return springs or something inhibiting motion of the spool.

Good luck because this is the stuff that drives you nuts. The quick check for the 3 ports is to insert a pipe plug and see if the aux compressor all of a sudden goes to a long time between cycles. That tells you the 3 port may have issues, but it does not rule out the corresponding 5 port from being suspect.

If your Norgrens are getting a little long in tooth (10 years or more) they are due for rebuild or replace.