Probably pine boring/bark beetles or whatever they're called. The buggers eventually killed the beautiful Shore pine in our front yard. Branches kept dying back and by the time I figured it out it was too late and the property manager cut it down, disrupting 22 years of our feeding all the birds that used it. They usually attack weakened trees and destroy the bark so they don't get the nutrients and die back slowly.
I think if you catch it early enough and kill off the grubs you might save it but it's probably done for.