By bowing in shape, do you mean it's starting to look like a collar?
I might have done this when I first started crocheting, only mine was a dishcloth. lol
It was due to the fact that I wasn't crocheting with the same tension on the the yarn throughout. I started off really loose, then as I crocheted along, my stitches got tighter.
With practice, you'll get it right! Good luck!