Many sharp objects, especially those that lie flat on the road like nails and pieces of metal, more often enter rear tires than the front tires. That is because the front tire upends them just in time for the rear tire to be impaled on them.

For example, nails seldom enter front tires. When dropped from a moving vehicle, nails slide down the road, and align themselves pointing toward traffic, because they prefer to slide head first as they would when laid on a slope. The front tire rolling over such a lengthwise nail, can tilt it up just in time for the rear tire to encounter it on end. I once got a flat from a one inch diameter steel washer that the front tire had flipped up so that the rear tire struck it on edge. When following another wheel closely, the front tire can get the "rear tire" treatment from the preceding wheel.

The front wheel set-up effect is especially true for "Michelin" wires, the fine strands of stainless wire that make up steel belts of auto tires. These wires, left on the road when such tires exposes their belt, cause hard to find slow leaks almost exclusively in rear tires.

When wet, glass can stick to the tire even in the flat orientation and thereby get a second chance when it comes around again. To make things worse, glass cuts far more easily when wet as those who have cut rubber tubing in chemistry class may remember. A wet razor blade cuts latex rubber tubing in a single slice while a dry blade only makes a nick.

As for pinch flats, aka snake bites , they occur on the rear wheel more readily because it carries more load and is uncushioned when the rider is seated. The rider's arms, even when leaning heavily on the front wheel, cushion impact when striking a blunt obstacle.