It takes X PSI to inflate the air bags to a certain level with a certain amount of weight on the trailer. If you remove the weight very fast, all of the sudden X PSI is able to inflate the bags out to a longer length. Regular leveling valves are not able to release the pressure fast enough in some situations. The risk is that you can stretch the bags, make them burst, etc.

Some here will think it's a joke to dump them, but IMO when you're doing something such as unloading hay, unloading grain over a large pit where you can dump in terms of seconds instead of minutes, it is a good idea to dump them. I don't bother when unloading in the swing away auger that takes 10 minutes, but I do when I'm at the ethanol plant where I can dump in under a minute. Not worth risking it for being too lazy to flip a switch IMO.