Plasmolysis in plant a cell occurs due to:

Exosmosis is a process due to which the water molecules move from a higher concentration region of the cell to a lower concentration region of the surroundings through the cell membrane. 
When a cell is kept in a hypertonic solution, it shrinks and exosmosis occurs.
Plasmolysis, in a plant cell, is defined as the process in which the cells lose water in a hypertonic solution.
Plasmolysis is the shrinkage of the cytoplasm in a hypertonic medium.
So, Plasmolysis in plant cells occurs due to exosmosis.