Without knowing the kind of plant you have and how it's been cared, there can only guesses. These are 3 possibke causes which might happen to your succulent:

1. It is gone seasonal change. This happens for certain succulents, that during some time in a year, they will go a period of dormancy. When this happen, these succulents would have the leaves fall and look dead.
2. It is adapting to a new environment. This happens if you have just bought the succulent or you move it to other place. As a plant, succulent can't adapt by moving itself, instead it will adjust its own growth according to new environment. In this case, the adjustments might include leaves fall.
3. The plant is dying. This can be caused by several things. Pests, rot, fungi infection, or mistake in care (ex over/underwatering). In rare case, the plant might've reached its age and growth limits, and begin to die of old age.