Too bad you don't remember what site it's from, but it looks like that graph is not quite right for the network. You can check other sources for network hash rate estimates over time, e.g.:
https://blockchain.info/charts/difficulty?timespan=2year&showDataPoints=false&daysAverageString=1&show_header=true&scale=0&address=and you'll see that the change is much more gradual, which fits with ASICs coming online (which, yes, are certainly the main cause of the curve going the way it is on proper charts).
Perhaps it's from a specific pool, though? In that case, the jump could be because more miners joined that pool. There was some concern about Ghash.io getting to be too large, so a lot of miners changed to different pools at that time. That was in mid-2014. I'm not sure if the picture you've added showing a range from january to july is actually showing that range (in which case that would fit)... or hours around the 17th of january (in which case.. no idea.)