Aspect ratio jumps on first few frames

I have done a Sony Vegas 8 multicam 3 camera project, all cameras supposedly using 16:9. 1 HD cam, and 2 SD cams.
I noticed the footage from the HD cam was slightly different, and set each segment to "match output aspect" and sometimes used pan and crop to re-frame.
The final DVD has a strange habit that the first few frames from each of these adjusted segments were larger, and then the picture would jump in to match the pan crop settings I had used.

Is this some type of GOP problem? has anyone seen this before?
Chris

Thanks for that hint, Yes, there is only one keyframe.
It looks like the problem is related to the "Quantize to frames" setting.
Once I had set that on, then problem resolved itself.
I suppose I should RTFM

Chris