Causes:
The centre of the tread bends inwards in the footprint area due to a low pressure, therefore the tire runs mainly on the shoulders This increases slipping on the contact surface & causes more rapid wear on the tire shoulders.
Tyre fitted on too wide rim
High lateral thrusts (for example with rapid steering actions on roads full of bends practicable at high speeds).

Reccomendations:
Pressure checks, including the spare tire, must be made every two weeks using an accurate pressure gauge
Fit the tire on the proper rim.