That improper manufacturing could allow the nut to detach, separating from the pinion. If that happened, the driveshaft and rear axle could become detached, possibly from the vehicle altogether. That could also cause the rear differential to fail, locking the rear wheels which is an obvious hazard that could easily cause a crash. The recall includes 2009 and 2010 model years for both vehicles.
The recall has only been announced in Canada so far, but will almost certainly progress to the U.S. soon in far greater numbers.