Ghost.. it means that the failure occurs on a “ghost” request. It is simply a random radio signal that was received by the BTS, looking like a RACH access. The BTS process it as a “real” RACH request, starting a SDCCH assignment.
The SDCCH assignment fails because the RACH is actually a “ghost”… there is no answer from the Air interface after the BTS sent its AGCH messages.
To reduce it, you can increase the RXLEV ACCESS MIN threshold. Frankly, as long as it doesn’t induce RACH congestion, you shouldn’t worry too much. Keep the SDCCH failures due to ghost within reasonnable amount (< 10% for instance).