At least nine people have died from storm-related injuries from torrential rains that fell over the weekend, a lot of them from falling trees. As for why the trees fell, gusts were reported up to 70 mph in some places. Add in the soaked soil from inches of rain that fell and you will find places where the otherwise solid ground is loose. The roots aren't held in place as well. Get a big gust of wind and it increases the chances for a tree toppling.
The rain should be ending today though. The winds have diminished too. And thankfully, the rest of the week looks dry.