Towards the end of 2016 the rate at which new users were joining shot up:
I can speculate (eg a major tidy of tags and Titles perhaps?) why but do not have a plausible theory that might explain both the jump and persistence at the higher level. ‘Hats’ may have drawn interest from other SE sites but this might only explain the increase, not that the higher level has been sustained.
Changes in Google’s algorithm have been known to make a significant difference to traffic, but I thought for SE that was generally felt as reductions (ref), and anyway was earlier in time, from what I recall (WA did experience a dip in the middle of 2014).
Google refines its algorithm very regularly, and I see nothing very exceptional at the beginning of November last year. Yes, there were “major updates” at around the right time but with 500-600 search algorithm changes every year there can be several “major” ones in a single month. Anyway, some industry speculation was that the change around November 10 was largely reversed about a week later. (Also: it takes a fair bit for Google visitors to ramp up, if SU, SF, and SO are any indication. – Jeff Atwood♦ Jul 29 '10 at 3:46.)
Better understanding may help with providing a better service, in particular to our main audience, the viewers who have no say here.
Is there an explanation - those with access to site analytics may have a better understanding? (Even a plausible hypothesis would be of interest.) Users who joined at the start of November 2016 might have some insight.