I just thought of a very real possibility, I think. I fermented out the starter over 4-5 days, because I was out of town. On my return, I put the starter in the fridge over night to drop the yeast from suspension. When it came time to pitch the yeast, I pulled it directly from the fridge, decanted most of the starter liquid, and pitched it into the 68F wort. Actually now, I pulled the starter from the fridge about an hour before it was time to pitch, not enough time for the ~40F starter to reach the 68F wort temp. Big shock, I think. I should know better.