Triggering "< PREV" button

I was Previewing the most recent project I've been working on when I discovered the "< PREV" button wasn't activating.  I realized there was no trigger in the slide for the "< PREV" button, so I added a trigger for it.  Once I did so, the "< PREV" button navigated to the prior slide.  That was all well and good.

But I began to think that I couldn't recall adding triggers for the "< PREV" button in projects I had developed before.  I checked the courseware in production and found that the "< PREV" button functioned as intended.  So I opened the project I had developed before and verified that I recalled correctly -- I had not added triggers for the "< PREV" button.  I had only added triggers for the "NEXT >" button.

Why does the "< PREV" button navigate to the prior slide when the user clicks it in the projects where I did not add triggers for the "< PREV" button?  I used the same template for all the projects.  What could be different about the most recent project that would make me have to explicitly add triggers for the "< PREV" button?

Update: OK, I think I figured this one out myself.  The  "< PREV" button works without an explicit trigger (assuming you have a trigger for the "NEXT >" button) if you add no other explicit triggers for the "< PREV" button to the slide.  But as soon as you add a trigger on the "< PREV" button somewhere else on that slide, the "< PREV" button will no longer take you to the prior slide 'til you add another explicit trigger for it.