When you're over watering your plants the plants leaves will start to wilt and fall over or the leaves of the over watered plant may also fall off.
The over watering of the plants causes root rot which then leads to the plants becoming dead or near death and so the leaves fall off.
If your leaves start to turn yellow or brown then you're likely over watering the plants.
When watering plants just use enough water to make the soil wet and then water again when the soil starts to become dry.
You should also use a pot that has holes in the bottom and when water comes out of the bottom of the pot then you've given the plants enough water.