The 2003 article, Infant Heads: Too Big, Too Small, Misshapen, by Joseph H. Piatt, Jr., MD explains the mechanics of skull flattening as follows,
The mechanical difference between an infant's head and a water balloon is only a matter of kinetics. A water balloon laid on a table conforms to the contour of the table instantly. The infant head laid on a firm mattress takes weeks to conform, but if the infant is sufficiently immobile, the result is the same. (p. 3)
You see a flattening of many infants heads for any number of reasons. Two of the most common reasons are the 'Back to Sleep' campaign and congenital torticollis. Another reason is the fact that we are a very mobile society, and as a result our infants are very immobile. This limits the amount of time they get to spend on their tummies and instead places them in their carseats, infant seats, bouncy seats, or swings-----on their backs for a large portion of the day. As parents, we need to take advantage of those wakeful moments and place our babies on the floor, on their bellies for some 'rolling around time'! If a child has torticollis, or a tightening of the muscles on one side of the neck, we must be even more diligent about giving our babies time off his or her back.