How do I change the default behavior for when a viewer doesn't make a choice?

You can change switching behaviors in the Edit Overlays screen for any clip.

If you want to auto-choose the next video, use either the Instant Switch or Seamless switching mode. Instant Switch will jump to the next clip as soon as the viewer makes a selection, while Seamless will wait until the full selection window has passed before moving forward. The auto-chosen video will be the first video you connected to the clip, and will be visible at the top of the overlays list when you are in the overlay editor. 

Loop mode and Wait mode will require the viewer to make a choice before the video continues to the next clip. Loop mode will loop the clip continuously until the choice is made, while Wait mode will just freeze on the last frame until a choice is made. If you need to change the default choice on an existing clip, you need to remove and re-add the connections so that the desired choice is at the top.