Summer Branch Drop is the failing of mature tree branches over the summer period. There has been little research into why a mature tree would suddenly shed and drop their branches but some belief with the onset of heavy rain followed by dry hot spells the branches become overloaded and stressed, dropping to the earth. There seem to be certain trees which are more susceptible than others, these being Beech, Oak and Horse Chestnut.
I have personally seen and heard this natural phenomenon happen and the tree gave out a warning sound like that of a shotgun or sharp loud crack before a very large and heavy limb came crashing down to the ground below.
The best precaution you can have against this is to have your trees regularly inspected and if the branches become overloaded, and the branch and leaf distribution is very uneven then a reduction would be advised to help the tree re-balance itself and have a healthier life span for all involved.