I wonder if it is a phenomenon called 'summer limb drop'. I discovered an article that says that some tree types, including maple, are susceptible to this, particularly in July (and, even more specifically, between noon and 4pm, and in situations with little or no wind!). This is apparently quite common and there is no clear cause (though it may be something to do with water stress).
The suggested remedies are preventative:
Most 'branch drops' are of long horizontal limbs, so it is recommended that you remove or shorten and lighten any long horizontal branches.
You can also reduce the branch weight by thinning the foliage, or cut back the long branches to an upright branch closer to the trunk.
I don't know if this helps much - maybe someone else can come up with better advice. Otherwise you could see if Mulder and Scully want to take on the case as an 'X file'!
Last edited by starfish; Jul 30th, 2007 at 10:48 AM.