As others stated the problem is caused by the natural movement of moisture out of a home and compounded by improper ventilation.
Might I suggest you go to www.Airvent.com, look for the Principles of Attic Ventilation manual, download and study…
Now take what you know from the manual, do the math for the home and compare what it should have to what it does have…in my experience dealing with roofs and attics I think you will find the answer is obvious…the builder didn’t understand ventilation and therefore was not about to waste precious pennies on things that ‘don’t really matter’ like baffles in each rafter spacing, soffit with the correct NFV, and a ridge vent with an external baffle.
These problems too often go un-noticed such as in this case for 10 years…long past the homes warranty. Often when found the repairs made are only what is needed to nail on a new roof because the roofer is just that a roofer not a roofing and ventilation specialist who is brave enough to say your home was not constructed properly and that is why this happened. To do this breaks down the warm fuzzy feeling that the homeowner had about the builder and makes the bearer of the news a (insert your own word). The news and information given by the person who does know what the problem really is challenged by the homeowner, the homeowner then seeks out family members, clergy and people in the workplace for their opinion. Upon finding one person that put a roof on something or at least had a brother in law that put on a roof or two that assures them that it wasn’t done this way 30 years ago and that a shingle is still a shingle. The bearing of the truth is now chastised and run out town on a rail until repairs to the roof are made without corrections to the ventilation…Until one day the homeowner is in the attic looking for something and sees the problem again…and the SOB that told him the truth he didn’t want to hear or spend money to correct gets to say I told you so.
Sorry I rambled…just some flashbacks…just educate yourself about attic ventilation and be prepared to have your findings and recommendations challenged…by the un-informed.