Atmospheric conditions are responsible, mainly moisture and wind.
You are witnessing one of the oldest forms of storm forecasting; leaves on trees turning upside down.
The air preceding the storm is moist cool air.
Leaves on most of the deciduous trees in your yard react to this incoming moisture and wind pattern changes by seemingly turning over. The moisture or humidity in the air settles on the leaves making them more pliable and softer. When the wind speed picks up, these leaves will be lifted up and turned in all directions exposing many undersides in the process. As the sky darkens, the undersides of leaves become more visible and the mosaic patterns of lighter green mixed with the darker green becomes more enhanced.
Leaves on trees turn over or turn backwards just before a rain storm due to increase in humidity. The turning over of leaves increases their capacity to absorb rain water because the stomata now faces up. In ancient times, seeing the undersides of leaves meant that a storm was approaching.