Second thoughts - what about the thread tension as it was
going trhough all those layers of fabric?
How do your stitches look, are they smaller than you think
they should be?
The next question is fabric quality?
After that the bobbin tension or tensioner setting.
Stitch length:
I usually sew at 2.0, most people sew at 2.5 or above
with some fabrics my 2.0 stitch length can cause rippling
or waving. I spray starch the pieces which usually eliminates this issue (sometimes).
I just don't think the length of your border would cause
rippling or waving as I always cut my borders long in case of a boo-boo as I would rather cut ends that have to recut new borders as I usually don't have the material.
Hope this helps.....