It happens guys. Same thing to me. letter of employment signed, fleet assignment (DEC) and after a few days and after resignation from current employer everything withdrawn with no reason.
Everything was ok with security check, I complied with all docs requested.
The sad thing is that human side is completely absent. Nobody cares about leaving you jobless. And even saying that you can reapply in 12 months seems weird if you don't know the reason why the cancelled your employment.
Strange policy....