the champion audi had some sort of clutch/gearbox problem. if they had been able to get it around the circuit, they probably could've done the rear end change, but it couldn't, and theys could not even make some patchwork repairs to get it to the pits.
SV.com speculates there is some problems with software the runs the tire pressure sensors recently put on the audi interfering with the software which controls the gearbox