Dropped call but Normal Call Dlearing

I was tracking a series of phone calls that dropped while I was on them. As I was looking at the CDR’s it showed ‘Normal Call Clearing’. Can someone explain what could have gone on to not register these as a drop?
I believe that it could have been the customer hitting the end key, or the phone malfunctioning and sending the termination message. But could this have been caused by a parm causing the disconnection, for example max timing advance or exceeding rx qual?