why is ozone layer getting depleted at the higher level of the atmosphere ?

Ozone is present in the stratosphere layer of atmosphere. Mainly CFC's (Chlorofluorocarbon) are responsible for its depletion.  Chlorofluorocarbon are a group of chemicals including carbon tetrachloride and chloroform and are commonly used as solvents, aerosol, propellants and refrigerator coolants. They are not readily broken down in the atmosphere and eventually they diffuse into the stratosphere. High in the atmosphere they are broken down by sunlight, releasing chlorine and fluorine. They react with ozone and break it down into oxygen faster than it can be reformed from oxygen into ozone. This depletes the ozone in the atmosphere.