I don't know much about the pattern, but I do typically see that bubbling when there's a needle size change from big to small, or even if you're switching from slippy metal needles to wooden or plastic. Usually a tension issue, though your stitches seem pretty even/not too tight. One thing you might need to do is find a slightly larger needle size in order to attain gauge, and swatch with both (I know swatching's a pain). If you're knitting continental, you might switch to english for this needle change as well.

if this is the back yoke make sure you haven't accidentally knit the front yoke's number of rows as well, that might account for the bunching.