I think jvs is on to something with the mugo pines. From what I found, the dwarf pines all share similar frailties. They are prone to insect damage, and this year saw the 15 year cicadias come out, and they are all prone to root rot. The soil must be well drained, but since the roots grow close to the surface of the soil mulch is recommended (perportedly to keep the roots cool)
It also sounds to me that they may have been planted too deeply. I am only guessing here, but it is something to consider.