rear end air suspension system on 2002 GMC Envoy

can someone tell me why my envoys rear end lowers and sometimes its ok.

Asked by for the 2002 GMC Envoy
Your air suspension is starting to fail. This is a very expensive repair. You should consider converting it to standard suspension. It is much less expensive and you will not be able to tell the difference. Google one of those Strutmasters kits.
I appreciate the help, thanks so much.