Why does the blanket im crocheting curve?

It might be one of two problems. You may be adding extra stitches accidentally which will cause a curve because of the added length. But most likely it has to do with your tension. If you don't keep the same amount of tension on the yarn as you crochet, then some stitches will be tighter and some looser. You may be able to fix some of the curving with blocking after you finish the project.