Weld Distortion is caused by two factors, the general uneven heating of the workpiece that welding causes, and the weld metal itself shrinking. This later cause makes sense, when a weld is created you are producing a moulten pool of metal, like most materials, this hot metal will be in an expanded state. Thus, as it solidifies and cools, it contracts, or shrinks. As the weld metal cools and shrinks it pulls on the metal around it, creating stresses and thus distortion.
Welding Distortion can be corrected by stretching the weld metal to relieve the stress caused by shrinking. For butt and outside corner welds, this can be done by peening. Do this by holding a metal block behind the weld and peening evenly along the weld with a hammer. Be careful to avoid over peening as this will overstretch the weld and re-distort the workpiece, this cannot be corrected easily!!! So peen the whole weld a little, assess the effect, then if necessary, peen a little more.