Here's a tutorial
on just what you've done but it's a little vague about showing the final knit across both sets of sts (in this case, a k2tog). You may be twisting the cast on sts which is making the join row slant. If you think that might be it, go back and unknit the row. Now at least all the sts will be on one needle. You can re-align all the sts onto another needle and then put them back onto the working needle or you can re-align the sts as you knit across the row.
(Jan, it's a pleasure to see your beautiful details on this hem.)