coverage
interference
path unbalance (poor tx or poor rx in the bts, usually, it doesn’t generate alarms)
in some vendors, the sdcch drop also contains the sdcch congestion, so check that out also
and finally, you can check the SDCCH average duration. If it’s higher than 6 or 7 seconds on most cells of your network, there might be an issue during the authentication phase (dialog with HLR)