Ponder dinked and dunked against the Seattle zone for a bit of first half success. He didn't do a whole lot of damage. Indeed, it is the pass interference, defensive holding past five yards, and personal fouls that have habitually bailed opponents out on third down during the course of the season. It wasn't any different with the Vikings. Seattle leads the league in pass interference penalties and the margin would be even larger if they played press coverage 100% of the time. I believe that the staff switches it up a bit when the refs tighten up the calls.
Regardless, with respect to the first half touchdown, Sherman was beat in man coverage on a double move. Another reason to switch things up a bit. Zone keeps the plays in front of you to avoid the big play.
The fore mentioned circumstances easily explain the rationale for using some zone.