While discussing the effect of a beta site, attention turned to accept rates on SO. There's a sharp fall in global accept rate (not per-user) in early 2015:
This is also true of Ask Ubuntu:
And Super User:
And of Server Fault, and to a less degree, Unix & Linux, but not TeX - LaTeX or English Language & Usage.
What happened? Or did nothing happen, and I'm reading too much into happenstance?