Prematurely cracking roof shingles are unfortunately quite common. While there are several different factors involved, and varying degrees of damage, in general the deficiency lies in the design of the shingles as manufactured, and usually is not traced back to a significant problem with the installation.
Modern asphalt-fiberglass shingles are designed to seal down to each other after they are installed, to help prevent blow-off in high winds. This causes previously separate shingles to function like a continuous membrane. With the extreme temperature changes that normally occur to roof shingles, resultant expansion and contraction of this continuous membrane tends to tear the shingles apart. The cracks therefore are usually fully through the shingles, leaving your house susceptible to leakage and damage.