Got hit with a large drop in the SERPs this weekend

My web design and development site has slowly been crawling up the SERPs for the keyword "web design" for the last six months. I've been slowly developing my SEO attack and refining it, while at the same time trying not to over-do. Last Monday (September 3rd), I broke the top 35 and had hit #29 by Thursday. (subject for another day: SEO "professionals" who claim that SERPs don't change on a daily basis).

I took a long weekend, and when I checked my rankings on Monday night (the 10th), I had dropped to #82. Tuesday the 11th I was #88 and today (Wednesday the 12th) I'm at #85. I seem to have been hit with some sort of nasty -50 penalty, even though I've played it pretty straight and jumped through all the Goops (Google hoops).

I'm patiently waiting it out - I have 5 or 6 other great keywords that I rank highly for that haven't dropped in the SERPs so the penalty only seems to be applied to the one keyword: web design. Maybe too many backlinks too quickly?

Or, was there an algo update this weekend that changed the rankings of certain keywords? The keywords I rank highly for have all been strong SERPs for a long period of time. Perhaps an algo change that has something to do with length of backlinks?