These are highlighted sequences and you can click on them. Hover over the dot to see the preview pop up and click on it. Even though the red dot looks like a bit off from any sequence, it actually isn't. You'll see this as you start playing the sequence. (We just didn't want to pile these dots on top of the green ones.)
I have chosen a few of these but can't take all the credit.
These highlights, to a large extent, represent technically great captures. That means that the photos have been taken close to each other, the camera has been relatively still, there are no objects (such as car hood) or reflections covering the view. There is also a random factor of us stumbling across those sequences, so there might be really good ones out there that we have not highlighted. That is why we encourage to let us know of great captures via #MapillaryExplore.