I was just sewing some log cabin blocks together and had this same situation happen. Some wavy strips as I sewed them.
I discovered that there was ever an ever so slight curve in the strip. The ruler I used to cut the strips was an old one. I picked it up not realizing it. The ruler was worn down and as I cut the fabric it created an unnoticeable curve until I started to sew them together.
This may also be a reason for wavy strips.