Anyway, what is somewhat troubling though, is that 2014 seems to be a year of decline.
About those 2013 releases, zooming in on C64 Graphics, we can see part of the big increase was probably caused by the many graphics compos, such as Font Creation Competion 2013, C64pixels.com Double Screen Compo 2013, Plain PETSCII Graphics Competition and the 3-Color-Logo Competition 2013. Here's a table with the top 15 "ReleasedBy" IDs as listed at CSDb (ranked for 2013, with the other years also in the table). As you can see, JSL was going strong both in 2013 and 2014 ;-) But you can also see that a lot of those in the top 15 of 2013 did hardly release anything before or after that year, hence the correlation with the compos in 2013.