Hi Lucka,
The reason of high call drop rate is due to the SD mean holding time, i-e lets say call gets SD from the serving cell and immedialty TCH assignment take places from neibor cell (depend on the AWOFFSET Value) due to congestion. But if the signal strength gets drop on the neigboring cell it will get back to SD of the serving and if by that time SD mean holding times gets expire the call will drop.