Hard to say without checking the whole shot, but this usually happens when there is no background for that particular section of the road available in time to use amongst the frames that isn’t covered by the foreground matte.
In this instance, if the foreground shape is always covering the part of the road at other points in time, Mocha assumes that you don’t want to use the background as reference underneath that area.
In some cases you may not have this information, especially if the background disappears from view before it is car-free.
Having said that, I think there may be a solution here. If you’re willing to share the shot to my email, I can show you how to do this: martinb [at] borisfx.com