We get many calls from tree owners worried about trees flexing in the wind. Conversely we get no calls regarding trees that have been planted too rigidly, so they cannot flex.
Flexing sends a message to the roots, that they are required, even to the point of where they are required (i.e. prevailing wind). Without this stimulation roots have less incentive to extend.
This has the by product when it gets drier, there is less of a root area for the tree to garner moisture.
It is when trees do not return to vertical or if the trees are dead and have no flexibility you should call an arborist.