1. Saturated Soil
Soil anchors trees into place, so when it becomes saturated, tree roots lose their grip. Furthermore, urban trees can have very shallow roots growing in small soil volumes. Combine this with a heavy rain-saturated tree canopy, a bit of wind, and the tree falls over
2. Roots Lifting from the Ground
Speaking of roots, check if there's a shift in the tree's root plate. When a tree starts to lean, the roots on one side lift out of the ground or create bulges in the soil. If your tree is suddenly showing these signs, caution off the entire area and get it inspected.
3. Dead or Decayed Wood
Wood that's dead or decayed is weak and breaks easily. When subjected to rain and heavy winds, you have a recipe for tree failure. Decay fungi also love wet weather, so if you see A LOT of mushrooms popping up around the trunk base of the tree, that means even more decayed wood. Caution off the area and get it inspected.