The lean in a brick chimney is usually caused by driving rain, wind erosion and rapid drying out on one side of the stack - in this case the south, (right hand side).  The other side of the stack expands as a result of condensation and sulphate attack from flue gasses on the inside of the flue while, on the outside, the mortar between the bricks expands due to sulphate attack.

They were a common sight but are now seldom seen because when people stop using solid fuel there is no practical reason for keeping the chimney.  So rebuilt and especially rebuilt leaning chimneys are rare.