Quick leaf drop could be due to environmental issues; drought or root rot from to much water, drops in temperatures could also cause leaf drop.
A fungus disease could also cause leaf drop.
Drift from a Herbicide can also cause a quick death of plants.
Adjust watering if you determine this could be a cause.
Inspect the plants for signs of disease or insects. If you suspect this as a cause you can treat with Neem Oil.
Neem Oil works as both an insecticide and fungicide. It is safe for people and pets.