I always assumed my penalty was because I traded with around 200 sites over a 3 month span. But the fact that my site has been gaining 200+ backlinks A DAY naturally over many years makes me believe I had enough built up strength that google wouldn't care about those additional trades.
One other possibility could be that one day I decided to add "nofollow" to all outbound links. My site is a directory of sorts and we list categorized sites daily. Each of my 50+ category pages lists about 30 of these outbound links to other sites that we approve. I nofollowed all of them on every page. So in one day about 1500 outbound links were nofollowed.
Could that have really screwed up my overall site link profile in the eyes of google? Enough to trigger a penalty? This penalty didn't happen within days of doing this... more like a few weeks after.
thanks for your thoughts..