The reason given was that Perth had a high concentration of FIFO mine workers who had a high rate of positives on the ETD scan. Also the miners were, as a general rule, a belligerent bunch.
I could certainly see that happening. The use of the electronic randomiser selection should now remove any question of being deliberately targeted. Unless of course you go through at a quiet time then your a monty to be rubbed up by their grubby little swabbing device.