People who upgraded to IE7 were capable and willing to upgrade, and more likely to upgrade to a later version when available.

IE8 was the included browser with Windows 7, which was generally well received compared to Vista which included IE7.

The timing lines up within the release window of IE 8 (March 2009) and Windows 7 (July 2009), so potentially a lot of upgrade push from other things in the air at the time.

Edit to add: people on ie7 because it came with Vista probably had a more capable/pushy update system than those on XP with ie6; but I don't remember the details on how microsoft system updates worked at the time.