CDJ Crash while switching from Traktor HID to USB

Two weeks running now I've had djs come on after me and effectively crash the Cdj.  They plug in their USBs and switch it over to USB mode, when my laptop gets unplugged the deck stops play altogether, and then reboots. The club is heaving at this point, management don't like it.

I'm running Traktor on a Mac 15" 16gb using HID mode. The club has cdj900 Nexus decks and djm900 nxs mixer.

what are we doing wrong?