The PERS-43 brief, from Tailhook 2013, is very informative.
http://new.livestream.com/wab/tailhook
You will see a graph which shows the aviation inventory for each year group. There is a change from YG04 to YG05; around 100 less people. Though it is not much, the selection rate for O-4 should be better next year. As mentioned before, the percentages being low may be partially due to the 8-year commitment for all 1310s.
Also in the brief, there is a slide which shows the control grade URL. Aviation is significantly overmanned with the O-4 billets. All the other communities are undermanned in filling O-4 billets. Does this have anything to do with low select rate for just aviators? Wish I knew the answer.
If anyone wants to skip all the other stuff, about the 14-minute mark is where the last O-4 board is discussed, and the shifting of personnel in the zones.
No matter the justification for the low percentage this year, it was still brutal. Lots of good guys I know had EPs, followed the career path, and still not selected. A few of them only chose their second and third tour based on what was a known path of success. Luckily, for the unfortunate, the legacy airlines have been hiring lately.