Service Issue At MB Dealer

Looking for some help and/or information from people who have experience with Mercedes vehicles.  I took my car in because for alerts including "Brake Wear" and "Tail Light/Brake Light Out".  They found the brake wear alert to be from a sensor and did the repair.  The lights were out and the whole fixture had to be replaced along with a firmware update.  They did all the repairs and told me to return for pickup.  It's a 2007 ML 320 CDI.  

Here's where I haven't gotten good information from the service rep.  When I went to pickup my car, immediately, there was a brand new warning saying "malfunction."  They took the car back, ran diagnostics, and found a problem with the AirMatic.  I understand parts wear out and have to be repaired, but I don't understand how it could happen while the car was in their hands.  Is it possible damage was done in error while doing my other repairs, when putting the car on the lift, or when they were driving it on their lot?  They have told me it's "wear & tear", but I have asked for a better explanation.  I want to know how it could happen while they had it.  It seems hard to fathom that a problem like this would happen while in their possession without some sort of error on their part.  Additionally, I have concerns about their attention to detail and thoroughness, because they had me return to the dealership saying the car was ready when clearly there was a problem.  Any info would be appreciated.  Thanks!