It originally comes from the IE Blog in, I think, the IE7 release cycle (my memory of the date could be wrong; it's certainly a while ago!). At the time almost everything there was pretty censored and overly-dressed-up.

To be fair, if I'm not mistaken, those comments dated from a time when most of your efforts were put into fixing ancient bugs and implementing CSS 2.1 and the like (which, uh, was years away from being a REC), rather than anything particularly volatile (well, ignoring things like display: run-in which were ultimately dropped at CR). From everything I heard and the order that was chosen, it ultimately seemed like what was probably the sensible route to catching up, where the order was ultimately aiming at getting an increasing baseline working, and spec stability was just one of many metrics used. I was definitely told that the line was mostly trying to spin things positively, though. Certainly there were plenty of things that weren't implemented as "unstable" were all but done, such as the majority of Selectors Level 3, of which 8/9 only supported a small subset. (Heck, were you even on the IE team that far back?)

There again, I could also poke fun at Flexbox and http://w3cmemes.tumblr.com/post/26637660418/believe-it-or-no..., given we all thought it was stable. :)

Perhaps the "bleeding edge" is a touch too extreme for what ultimately was wanted, but certainly something much closer to other browsers than where IE had been before.