Knitters and crocheters face this problem with their edges curling under and over and out of control. In order to solve the mystery, you must understand that different stitches have different structure that causes them to have distinctive characteristics especially in knitting.
As knitters you can understand the curling effect and how to either use it or avoid it. The most common reason your edges curl is due to the fact that knit stitches are slightly smaller than purl stitches. So when working in stockinette stitch the purl side pulls the edges of your scarf in towards itself and you get a tube instead of a straight piece. When working a sweater or hat your bottom edge will curl upward. This is sometimes a design feature but it's not for everyone. To solve the problem you have several edging options that can add depth and weight to your project and flatten out those curled edges.
Seed stitch border- 2-4 inches on bottom and at least and inch on the side
Ribbing-2-4 inches on bottom and worked by garter stitch on the sides for at least an inch
Crab Stitch- for collars and smaller openings the crochet edging helps to add enough weight to pull in that curl.
There are other options but these are a few of the easiest to implement. If you need help just stop by the store and we can offer our ideas.
Crocheters tend to have this issue at the beginning of a project. Typically it will solve itself as the project gains more weight and structure. Often when the problem seems to continue, the crocheter should examine how tightly they are working. You may need to go up a hook size or two to solve the problem. If you are doing Tunisian crochet then it has to do with the way the stitches are formed and you need an edging stitch to help decrease the curling edges.