What did I do wrong?

Ok so we bought a camper today and brought it home. Towed great and the expedition sat level the entire ride. Get home, back it into my driveway and start unhooking it. I raised the whole assembly up to take some tension of the weight distribution bars and took them off. I lowered everything back down to level and unhitched the ball. As soon as I did that, the whole rearend of the expedition dropped to the ground. I mean it was like it had no suspension at all, just dropped straight down. I'll admit, I was shocked. About 5 minutes later it started to level back out and now it's fine. What happen and how can I prevent this from happening again? This is my wife's main car as I have 2 of my own that I use so I just want to insure I never see that look again (shuddering just thinking about it). Any help?

If you have the (optional) self-leveling rear air suspension, which I sure hope you do based on your description, there is a specific sequence for attaching and disconnecting a trailer.

I don't have my owners manual handy, but its well covered in the towing section. The process includes manually turning off the system using the dashboard controls attaching trailer, setting your WD hitch up, then reactivating the system. I believe cranking the WD bars -after- the air suspension levels out will cause you problems.

Also note the compressor will run for a certain time after the vehicle is turned off. So if you didn't deactivate the air suspension but turned the key off, the truck will still try to settle and level while you are attaching/disconnecting the trailer. This is why turning the system off is important.

I turned it off when I first hooked it up and set my hitch up. I had turned the expedition off after I backed it in the driveway and before I unhooked the camper. I was thinking that was my mistake. I'll leave the truck running next time and see if that prevents that from happening. Are you saying I need to turn the air suspension off before I unhook it as well?

I'm thinking that when the jack lifted the truck and trailer, weight on the rear wheels was reduced, so the suspension pressure probably was reduced automatically. When the vehicle was disconnected, the weight of the truck returned with the reduced pressure, so it sank. The pressure must have automatically adjusted to again only after you restarted the vehicle to return the rear of the vehicle to the proper height.