It's because you have the STB in Native mode. When you launch VOD, the set top tunes that VOD trailer channel and displays it in that scaled window on the top left. That VOD channel is 480i - so your TV changes to 480i (because you have it in Native mode).
You can enable Promotional Video Lock under parental controls to prevent the tune to the VOD channel, and the TV won't re-sync to 480i when launching VOD. Or - you can disable Native mode and be sure SD Override is off, that will prevent the re-sync to 480i also.