There are various causes for this, the plant discards the leaf, the plant cannot afford to sustain the leaf, the leaf gets damaged too much.
The first cause, the plant discarding the leaf: Plants may remove all the chlorophyll from the leaves, in order to prevent them from loss ( leaves fall during autmn because the plant does not want to spend the energy necessary to maintain them. The plant removes the expensive chlorophyll and stores it for later use)
The second reason is found usually in tight concentrations of plants, like forests. The leaves on the lower branches, which receive little sunlight, may not be viable to sustain. These leaves get discarded (and start to wilt)
An example of the third type would be plants growing in really salty environments. The plant may try to transport all the salt, which is harmful to the plant, to leaves which are deemed expendable. These leaves die of a overdose of salt.