Well,first off how long has it been since you had your tree pruned?
Is it near powerlines?dose it get pruned for right of way?
Has it ever been topped?
These are all key things to take in to acount,My random guess from doing this most of my life,is that the tree was topped at some time.When this incorrect form of pruning is done,it allows the tree to form realy tight croches,or branch attachments.The tighter the croch,the weaker the branch.
I would get a CERTIFIED ARBORIST to give it a look.
Call them up and tell them you have a tree you want a price on to prune,ask them about the problem then.They usualy do this for free,It would be a safe bet to have him prune it.