Aspect ratio shifting at start

This is an odd one to explain so the best way is probably just to show you:

In the first second or so the video always starts at 4:3 and shifts to 16:9. I've no idea why it could be doing it, but it seems to be a consistent problem. I don't play around with anything that might make it do it. Any suggestions would be welcome :/

is this just an artifact of the playing device I wonder? Does it happen when you play the orginal on other media players?