Curving?

So I started working on a new scarf, shown on the right.

Cast on 19 stitches

R1 - K
R2 - K
R3 - K
R4 - K2 *y/o, K2tog (repeat 8 times)* K1

I like the pattern it creates, but I'm having a minor issue. The further along I get, the more I'm noticing that the scarf is starting to slowly curve. I'm wondering if it has anything to do with the pattern itself or maybe the way I'm knitting. If any of you have any ideas on why it's happening or how I can stop it, let me know.