Hi Jonathan,
Welcome to the gray SEO world, full of "ifs" and google actualization. Is very hard to tell what happened, there is a lot of factors.
Google is change the algoritms again, and some points as load speed and text in the beginning of the website are becoming important.
And there is also the action from the concorrent websites. They got more links? they changed the website?
One tip I can give to you is stay connected at SEOMOZ blog to see the last changes at google and get a change to be ahead of the other websites.