I've ridden well over 600,000 miles and had dozens of rear punctures, but only one in a front tire. The same pattern appears to apply to the cars and trucks I've owned. If you search the internet, the explanation is usually that the front tire flicks a nail or screw up, and occasionally the angle and timing is perfect for the object to puncture the rear.

I'd think that a mud flap might help in some cases, but definitely not all. I won't mount one on any of my bikes because I've heard of many cases where the fender was damaged either from the flap striking the ground, or from the mounting holes. I'll take my chances and carry a pump and some tire plugs.