Are they newly planted trees or established ones ?   One of the causes could be a root fungus called Verticillium Wilt.  There was a lot of it about last year and it killed about thirty of my ninety two newly planted trees.  The symptoms are discolouring leaves which then crisp and drop off.  The branches then start to turn brown.  This is caused by the tree trying to defend itself against the attack by sealing off the water bearing tissues so dieback is from the top down.  You can read about it here.
http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=255
There is no know chemical control but although the article suggests uprooting and dicarding the plant, I've had some success with removing the dead branches below the brown bit i.e. cut through the green bit below.  Do follow the hygiene instructions though and use sharp secateurs and disinfect them between cuts with alcohol (from the farmacia) or even vodka (probably cheaper.)  I'm no expert, just passing on my experience.  You might be as well asking advice at you local co-operative.