Improper toe will affect both tires the same. Rather than rolling parallel and straight ahead, the tires are in a "wedge"-- the left tire points slightly to the right while the right tire points slightly to the left. The net effect is that the car still goes straight down the road, but in the process the tires must drag sideways over the pavement and they wear quickly.
This can be brought on suddenly by bending something through hitting a curb or a pothole with either side.