Originally Posted by Lori S

When sewing strip sets it is best to alternate the direction that you start the next strip. Sew one on at one end the next at the end you just finished at. When you do this you alternate which fabric is on the bottom layer. The feed dogs will slightly gather ever so slightly the bottom layer causing a curve . When you alternate at each strip , each strip has its turn on the bottom... so all the strips will have that same slight gather... and be straight. I have been successful in avoiding alternating by starching my fabrics prior to cutting... it seems to ride the feed dogs better, and no curve.
It can also happen if your cuts are not on the straight of grain... and there is a ever so slight bias edge.. this can stretch as it feeds through the machine. Another is that all layers did not feed at the same tension going through the machine. If the top or bottom layer is more tought than it will curve.