The worn rear tires have less tread and lower hydroplaning resistance. They can lose their grip on wet roads, causing the rear end of the vehicle to swing around out of control.
Deeper tread with greater hydroplaning resistance helps new tires grip wet roads. The driver can steer to compensate for the lower wet traction of the worn front tires and safely maintain control.
--dtc504-- Known in Oregon & Parts of California as America's Tire