usually, it is a drop that occurs when a hardware is lock or reseted, such as TRX lock or TCU reset.
If nobody did it, then it means that the software is doing it automatically. To troubleshoot this, you need RTAC I suppose.
Does it happen on 100% the cells of the BSC ? not in other cells of other BSC ?
The problem can therefore come from the BSC itself, AterMux interface or transcoder. I wouldn’t see anything else. You can’t find anything in O&M alarms ??