Spannersat KL is 100% correct. It is the ovalised wheels creating an out of balance as the wheels slow down after takeoff, until the retract braking kicks in.
When the wheels are extended the heavy part of the wheel falls to the bottom. When it makes contact with the runway the same part of the wheel is the first bit to be worn every time and it just gets worse and worse until the wheel is changed. The airline I work at checks for this at every 'A' check and changes wheels usually with lots of tread on one side but not much on the other.
