Well, having said that a new Doctor probably doesn't affect ratings much, I don't have an alternative theory for the 2 million more viewers who watch the 4th Doctor's debut story, "Robot" and stick around for at least the first 3 weeks.
Season 12 begins on the 28th December 1974, a whole 29 weeks since the finish of Season 11 on the 8th June. It introduces Tom Baker as the 4th Doctor, and for the first time in Doctor Who history, the new Doctor has been seen on-screen for a brief couple of seconds before the Season break. Perhaps this has added to the anticipation of seeing the new Time Lord?
Whatever the reason, ratings-wise Season 12 is very successful. If you want to talk statistics:
Generally speaking though, Season 12 is a solid ratings-winner, with a static core audience of just over 8 million - a noticeable improvement on Seasons 7 to 11.