The short-lived hub problem is caused by 2.7.1.0.
Check out the graphs at
http://crawler.doxu.org/history.html ... lower right quadrant. G2 started acting especially aberrant right around the start of February, but the orange "leaves per hub" line started trending downward much earlier, near the end of November 2013, a sudden departure from tracking very closely with the yellow line until then.
2.7.1.0 was released on Nov. 24, 2013.
I don't think that's a coincidence. I think something buggy in 2.7.1.0 made 2.7.1.0 hubs short-lived, or made them do something that affected the other hubs they were connected to, probably the latter (as I see short-lived 2.6.0.0 and 2.7.0.0 hubs). The orange line began dipping when 2.7.1.0 came out and perhaps continued to dip as 2.7.1.0 uptake continued, until some kind of nonlinear threshold was reached in February, perhaps when hubs could expect more 2.7.1.0 hubs connected than they could deal with due to whatever toxicity 2.7.1.0 emits to adjacent hubs.
Check the 2.7.1.0 G2 networking code, specifically stuff with diffs from 2.7.0.0 and probably specifically hub-to-hub (including hub-to-anything-including-another-hub), and I think you'll find whatever the problem is in short order.