That is so weird. Are there any ideas about why this happens?

I have a project going where I'll only need 3 strips, but it's something I can practice on since I have to make 13 sets of the 3 strips. I have never heard of that before.

I'm hoping with 13 strip sets I can hammer out a way to try and keep all that together. I've always sewn them in one direction so they are all even on one side.

That just seems so weird to me.

It is very weird !! It had me just scratching my head for a long long time. I think it for the same reason that if you put a fabric on the bottom the feed dogs just very slightly gather the bottom fabric, but thats just a guess.
I did notice it is not as likely to happen if you are sewing on all lenghtwise grain( both fabrics). it also does not happen if I starch well , and press after each strip. After the few major mishaps I've had with this issue , I do not chance it ... seeing how I just hate to unsew alot of strips.

I usually use strips that are WOF, but I starch the dickens out of everything.

Yep! some of us just have to learn the hard way. :lol: :lol: :lol: And sometimes I still need to be reminded!

Leave it (or put it back) under the presser foot as if you have just finished the seam, or that you are going to start sewing again. Use whatever method that is easiest for you to keep straight.

I heard once that this only happens if the machine has a little bit of a tension issue, it would happen. Of course, I didn't believe it and sewed my strips on the same direction. I ended up with a curved quilt top. Will not get caught doing that again.

Torque.

That's the technical term.

The dreaded "J" curve is what I call it.

I learned this the hard way. I had even seen the video where she talked about it.

I just didn't think it applied to my little bit of strips..it did. I had some serious wavy strip sets-which turned out ok because I was doing crumb type blocks so it added character.

I think the reason a curve develops has to do with the feed dogs pulling the bottom layer thru while the foot doesn't pull. The more strips the more it shows up.

If I'm sewing multiple strips together, I sew my strips in sets of 2, aligning the starting edge. Then I sew the sets together going in the opposite direction making sets of 4.