According to coinwarz.com chart, the total network hash rate dropped from more than 70MH/s to below 35MH/s on December, 13th 2016, right after 0.10.1 has been released. It smoothly recovered and is again above 50MH/s, but still far below it's highest levels above 80MH/s.
In the release notes I can't find any (technical) reason for that.
The network hash rate never reached 70 MH/s (I don't think it even reached 60 MH/s, though it came close), so this points to a bug in the site you're looking at. It seems likely that this is related to the switch in block target from 60 seconds to 120 seconds a while back. Some sites never noticed, and started reporting wrong difficulty. Maybe this site updated their logging at that time.
To see the "other side" of the problem, look at http://chainradar.com/xmr/chart, look at the difficulty chart, zoom out to "all", see the (false) bump from less than 1 billion to 1.9 billion on the 20th of march 2016, which is the hard fork that changed the block target.
