That could be one reason.
The main reason (or theory) that is often cited by arborists is that in the heat, a well watered tree, the branches grow well so fast in compared to its main trunk, that the rapid expansion of the bark's branches and the increased weight of the vigorous branch causes shearing stress at the attachment point to the slower growing main trunk and therefore will crack and fall off.
Another reason is that there could be previous cracks caused by insects, or weakening by fungi and when the branch grows, it will give way at the cracks or weak spots and fall off.
Still another reason could be the way the tree was pruned previously.
Drying of the tree branch is not the cause as drying will make that branch lighter, and usually dead dried up branches are the first ones to be pruned off or removed from the tree during maintenance. The summer limb drop on the other hand are perfectly growing branches, so green and often vigorous that suddenly fall off from the trees without warning, crushing cars or whatever happens to be below them.
