It is very common for tyres to be replaced way before their normal life time has been reached mainly because of rapid or uneven tread wear. Most common causes of premature tread wear are:
Low tyre pressure - Causing rapid wear along the inner and outer edges of the tread.
Inflating tyres too high - Causing excessive wear along the centre of the tread.
Defective brakes or shock absorbers - may cause the tyre to wear in isolated patches along the tread face.
Incorrect wheel alignment - Will cause excessive wear along the inside or outside edges of the tyres. If you hit the kerb or a pot hole - the tracking may invariably be misaligned which will result in this sort of wear. Wear in the steering components can also cause the wheels to pull out of alignment therefore it is a good idea to have the front wheel alignment checked at least twice a year or soon after a hard bump with a curb or pot hole.