This recent thread may be of help to you:
Need help with machine binding please!!!
The reasons I can think of are:
(1) the binding stretched while sewing, more likely with bias than straight-of-grain, but can happen with both
(2) if the binding looked fine before the quilt was washed and then wrinkled, it is likely that the binding fabric shrank
(3) if neither of the above, it is likely that you shifted the binding on the final step. In other words, if you had drawn a line on the binding straight across the binding (from edge of quilt to edge of binding), after folding the binding over, it got sewn so that the line is crooked.