Kelowna - 3:13 pm on Nov 11, 2013 (gmt 0)
September 4th - traffic drops to 90/day UVs and stays there.
I would say yes to coincidence. I have done this many times, it has never made any impact positive or negative.
I think google is still playing with the algo knobs, some of my sites shoot up, and others drop, rinse repeat every few days. Does not seem to have anything to do with +1 profiles.
From the Sept 4th date you give it sounds more like you have tripped a filter because something is over optimized, inbounds or onpage. I had a bunch of sites I had to change around that date. Even if you don't touch your site, and google changes, then your site can fall within what the algo considers over doing it. Compare your site to those ranking at the top to know for sure.