Laddering where the cord loop pops out is a tension thing. The cord pushes your knitting apart a bit and it's easy to end up with a bit of extra yarn between the stitches. You need to pull the two stitches after the loop extra-tight. You can also break up the column by moving where you pop out the loop every so many rounds to vary where the unintended loose stitches go. (Also, laddering will often shake out in blocking/the wash.)