As a Haworthia is growing, the older leaves slowly move to the outside where they eventually wither and die. For the quicker growing kinds, this can be a regular occurrence. Leaf loss can also be a sign of dormancy or protection. For some species like H. semiviva and H. lockwoodii, when subject to water loss, outer leaves will be allowed to wither to protect the growing tip. Water loss can be a result of root problems, limited water supply, or too much sun.
On the other hand, rot can cause leaf loss too. Unlike a withered leaf, a rotted leaf is often softer than healthy leaves and has a muddy color. Rotted leaves often indicate that the underground part of a plant is suffering from rotting.
When there are many leaves withered in a short period, or a rotted leaf is found, take the plant out of the container and examine the roots and the underground stems carefully.