Ghost RACH are random, so for each ghost RACH, the field “CAUSE” contains a random value. Can be 1111, 0011, 0101.. etc. If it was ghost RACH, you would see an equal distribution of each type of RACH cause.
therefore, it looks like it is LU from roaming MS’s. But why are they failed ? I have to re-read the 3GPP, but I thought a roaming MS is allowed perform a LU in a cell : SDCCH assignment, SDCCH allocated. And during the SDCCH phase (DTAP messages MS-MSC about authentication, etc.) if the MS cannot camp on this network, the LU will be rejected.
What I mean is that the rejection is not performed at the RACH reception (but thi is as far as I think !! this needs to be confirmed… anyone ??)