Has anything been changed in amongst these trees in question?
What's the age of the trees in question?
There are many factors that can lead to an accurate, yet sometimes hypothetical, prediction of the potential problems associated with trees.
Here in Manitoba, which is a totally different Zone (which for inquiring minds is 3), Paper Birches succome to Bronze Birch Borers. These beetles typically bore into the top 2/3 to 3/4 of relatively mature trees. Their feeding practices cause so much cambial damage that the top dies back and needs the care of man's hands. This is the same thing that Mike was referring to..........and even though there are many different chemicals treatments out there, they all are more effective than no treatment at all! I personally prefer a painted strip of Cygon 2e (systemic) located below the lowest lateral limb or at approximately 5' (done to prevent small children and animals from coming into direct contact with the chemical). This treatment gets all borers heading to the soil for winter hibernation and those that migrate to a new location from accessing the tree!
Birches are also very intolerant to high soil temperatures. These birches originally came from river and lake banks where the root zone is continually shaded and the canopies where in full exposure to the sun. Move this tree into an open yard with full sun on the canopy and the roots.............you'll soon have an extremely stressed out tree. Even though the use of organic wood mulch beneath the dripline will prove beneficial, the harshness of the sun may prevail.
The potential causes of death could be endless.........but hopefully some of this information will shed some light on your mystery!