As several others have said, I can attest to switching back and forth which end you start sewing from. When my quilt group and I made a quilt with lots of skinny strips sewn together we would alternate which end we started sewing from... sew 1 or 2 at one end, then turn it around and sew the next 1 or 2 from the other end...it really helps the bowing a lot!
Good luck,
Kif

When this happens to me it means I have cut too many layers of fabric at the same time. Usually 4 layers is all I can get by with then also you need to check to make sure the fabric is folded completely flat (sometimes when you fold several layers to cut you get a little wobbly where the fabric is folded) I always check after cutting about two strips to make sure I don't need to straighten the edges again. Hope this helps!

I think it is because the feeds dogs on the underneath piece have a tendency to feed the bottom piece in a little faster than the top. So, as you sew, pull on the fabric and keep them even. It is kind of difficult to explain, but if you don't hold the fabric a little tight, then after sewing two strips that started out the same length, won't end up evenly together...and I think that this causes the bending.

I would try lengtening stitch a little at time see if that helps

If you want to use these strip strata without resewing (most accurate) cut the strata in the center (highest curved part of the bow). This will help "staighten most of the roll when you are piecing.

Just remember, gentle hands when sewing, and check for staightness every couple of cuts. (yes, I mean this) It is easy to get off and hard to get back on. If you are off, cut a compensating strip to even out your edge.

I know that when you make a bargello quilt the instructions have you sew the strips together alternating which side you start on - start on top row and sew down for odd rows and sew from the bottom to the top on even rows to prevent this bowing. I now apply this to all my piecing and it seems to have worked.

If you do not alternate the direction when stitching these together they will bow. So one the first strip sew from the l-r, then the next strip sew r-l. This makes the world of difference.

when sewing multiple strips together you should reverse the direction you sew on every strip, if you always sew your strips starting on the same end, learned this in a bargello class where you make tubes of strips and cut them apart.
Also, someone else mentioned this, be sure that your strips are cut straight. Lastly, mentioned before, using a walking foot can help.
I found that starting on the opposite end after sewing the previous strip works.

Mine done that when I did not have the fabric folded properly before cutting my strips. Where it is folded is where you will get the wave if you don't have it folded even. I'm not sure I worded this right, but I'm sure there is someone on here that can, lol...this is where I learned that.