if nothing was changed RF wise, could be a problem with signalling back to core. Too much location updates may cause increased signalling traffic.
Maybe a bunch of filipinos decided to move into the area and started sending too many sms messages. hehehehe. How is the SDCCH Blocking anyway. Is it high too? I assume it is.. so check ghost siezures. if it is also high, run a BCC plan check.
BTW, in general, the way to reduce most SDCCH drops is the following.. proper power budget, BTS TX should be properly set (drops due to insufficient uplink rxlev in most cases), rxlevaccmin (drops due to insufficient C/N margin), and proper frequency plan (drops due to interference).