Well, we will never know but one of 4 things has happened.
He has finally run out of responses to Sebs speed and simply cant dig any deeper. Which is probably the case. OR, he is just not quite on it this round. Just like Button during most quali sections, and all other title contenders have had at times this year. This is the next likely scenario IMO.
OR, he choked...but in my eyes if he was going to choke he would bin it trying...I dont think choke = slow for Mark, it seems bin it with a rush of blood Or, for the conspiracy theorists, he is back there to play rear gunner for Seb....seems strange that he is down by so much in the section that is a drag strip. I know Webber was running a different rear wing on Friday and I put his lack of speed down to the fact that the new wing sucked. But assuming their cars are now on par aero wise he is still way down on the drag strip? Maybe Marks Brazil engine jsut doesnt have the goods in it anymore