you’ll need to identify which trx you lock (let’s call it TRX 1), and which one you keep on-air (TRX 2).
maybe your tch traffic goes on TRX 1 by default, leaving the TRX 2 empty most of the time (check TRX traffic distribution).
When locking TRX 1, all the traffic is forced to TRX 2. Maybe there are interferences on the frequency of TRX 2, leading to intracell HO. Unfortunately, it can’t go on the TRX 1(because it is locked).
So the call drops, due to poor quality.
When you unlock the TRX1, all the traffic can now go on TRX1 and be “saved”.
There are some other possibilities, but you need to detail more your problem (traffic / TRX before and after lock, HO split before and after, call drop cause before and after, tch mean duration, etc…) and the cell configuration (which TRX do you lock ? TRX “BCCH” or the TRX #2 ?)..