It happens...
Possible Causes of Bird Falling From Sky
Bird Biology
Bird’s unique biology allows them to fly, but also can cause them to fall. For instance if bird feathers become soaked in many cases they cannot maintain flight and will actually fall very quickly. This may be the case when large number of birds are found dead after a sever storm. Birds also fly between different air currents; they can quickly become frozen and unable to maintain flight. As is true with any creature there can also be natural causes; it is not uncommon for birds to experience health problems midair that result in death. Some scientists speculate the presence of an avian disease or disorder that causes birds to fall from the sky.
My first thoughts were a pocket of air much colder than the ambient temperature. Cold waves running along the ground have been known to kill herds of cows.