the SDCCH congestion may be due to the following cases :
1/ not enaugh SDcch time slot recommanded value is one SD/TRX .
2/ the cell is border so there is a lot of location up date ; check CRH and T3212 timer
3/ BCCH type combiner or not because there is two kind of SDCCH:
SDCCH/4 ( 4 sud channels ) SDCCH/8 (8 sub channels )
4/chech if there is a SMS traffic in this area
From my experience I’ve seen when other operator or roamers’ ms try for location update it is rejected, they again try and going on try. In this way SD is congested. There is a parameter called “PLMN Not Allowed” in MSC side which can prevent suck kind of behaviour.