Is there something in a Mac Pro that could make it sensitive and reboot when bumped?
This something is quite possibly a loose PCI card, maybe your video card, it can have some stress on it from the weight and pull of the VGA or DVI cable. Could be a usb daughter card as well. Unplug the machine from the power mains. Reseat the cards, blow the dust off them. Maybe it won't reboot on bump any more. 

tl;dr: A loose PCI card can cause a reboot, they aren't hot-swappable.