It could be one of two things and it always drives me nuts. If the balance of the tension is off, then over the course of several inches there is less bobbin thread being used than top thread, so the straight line of your edges becomes curved. What I usually do is to lay the strip on my table and then run my fingernail along the seam to pull more bobbin thread through the length of the seam; it seems to help. Also, if your strips are not cut exactly on the straight of grain it can result in the bowing. You're not alone.