To be simple, you can consider ghost rach as “useless” signals which are decoded by the BTS as “useful” signals.
So when receiving those “dummy” signals, the BTS will consider it is a “RACH” access and will start allocating a SDCCH for it.
Step 2: the bts has allocated the SDCCH and sends a response to the MS: “Go to this channel”. But since there are no MS (the original RACH signal was not generated by a MS in the 1st place !), there is no answer in uplink. The SDCCH is anyway “reserved” and will be deallocated after the expiry of a certain timer.
So the ghost rach can lead to sdcch congestion, if it is in very large amount.