If sitewide my Google Analytics shows a sudden and sizable decrease or drop in “Bounce Rate” why would that happen? How can I troublshoot causes for what’s obviously a tracking issue?
You should know out the gate that “Bounce Rate” in Google Analytics is not the magic metric that some thing it is. What it REALLY means is that a GA tag was not triggered a second time in the same session. This GENERALLY means that person LEFT the site by hitting back or hitting an offsite link on your page.
However, if you setup an event to trigger based on a BEHAVIOR then you can get a second TRIGGER fired, and that will diminish your Bounce Rate. In the graph above, we see someone deployed a Scroll Depth tracking event, which then triggers multiple events/TRIGGERS on the same page, killing the Bounce Rate metric.