Because you aren't squaring them off. Once a quilt is done you need to measure the sides and middle, which probably won't be the same, each side is normally different as each end. Kind of average them out. I'm not good at explaining it but there are great tutorials on squaring them up. The main thing is, when you put the sides on, both side strips will be the same size, you may have to ease the fabric in so that you don't end up with pleats and you do the same with the ends. If your outside strips are all the same size then your quilt won't be wavy. I hope someone can explain this better but this is your problem. A lot of people just sew a strip of fabric down the side and cut off the end, and they do the same with the end pieces, then you get wavy ends because they are different sizes.