Binding off the bands of a cardigan sweater?

So, I’m on the front bands of my cardigan sweater. Did 3 garter ridges and then bind off loosely. I don’t know what happened but after I bound off, it was all tight and did NOT lie anywhere near flat. I had to rip that whole section out. So, now I’m back to starting over with the garter stitches.
What am I doing wrong?

What was tight, the BO or the stitches? If it was the sts, you needed to pick up more of them; if the BO, try a couple needle sizes larger.

the BO was tight

Then use a needle 2 sizes larger and still keep it loose.