Be alert, hunters: Growl isn't casting first anymore, and it's making aggro touchy. Mania, that hunter maven and proprietor of Mania's Arcania, breaks the news. It's unclear (especially to me, since I don't have a hunter past level 26) if this is actually new or just a consequence of the new combat log system that rolled out in patch 2.4 but if it is new, it could be a nasty surprise for you hunters out soloing.
Mania goes over the list of reports, some arguing for better pet aggro, some worse, and specifically singles out boars as possibly suffering the most since growl scales with pet AP and the boar's charge ability adds AP to the pet's next attack. If that attack is growl, yay, more aggro! If that attack is not, boo, less aggro.
Being that I have access to a level 70 raid-geared hunter in the person of my lovely wife, I sent her forth into Outland to see what she could discover first hand. She reports that pet abilities appear to be casting in the order that they are on her cast bar, so she suggested moving growl to be first on the bar as that appeared to cause it to cast first. However, she still felt as though he was having a more difficult time in holding aggro for her than he did before. She also included a screenshot of her combat log.
Hopefully, if you're having this issue this suggestion can help. Mania also mentions that Hortus is taking bug reports on the issue, so if you're having these problems you should head over and give them as many details as possible to help them run it to ground and fix it. At this point, your guess is as good as mine as to whether or not this is happening to warlock pets as well: any news on that score from warlocks?