This is really only based on ~five minutes of messing around with SEM Rush, but it looks like some of your competitors have taken a little bit of a hit - to a greater or lesser degree. However, there is at least one actual competitor that appears to have increased their share quite a bit in that time.
If I had to hazard a guess, I would say Grossman Justice should be on your radar, if it isn't already. Yep. Sometimes it's just plain old competition.
Though I would probably revisit some of those guest posts on your site, done in 2012. They're pretty exact match anchor friendly. That might not be a terribly good thing. It wouldn't hurt to possibly nofollow those links.
Another thing that's a little strange and could be fixed easily is the 'Office Details' text on the contact page. It almost blends in with the background button. I don't think this is hurting much, it just looks bad. A little CSS tweak and you're looking good, at least on that point.
Though I would wonder who built thousands of links with the anchor text 'lawyer'. A lot of that happened hot prior to August 2014.
Without rambling on much more, I'm trying to say there are a lot of factors. Some of the little problems on your site, once fixed, can add up to bigger wins. Plus your competition has caught up to you. I don't think it's the plugin change, at least not at the moment.