OK,
The connection, as you know, can be carried by TCH as well as by SDCCH (SMS or LU for example), there are another data services which are served by the SDCCH as well. This requires the dedicated SDCCH to be used longer than in call setup procedure. During this extended use, esspecialy when the mobile is moving fast there might be a need to be made a handover to another cell. As the channel in use is SDCCH, the handover is made from one SDCCH to another.