Although, having grown up with keyboard-controlled games and the IJKL pattern, I was expecting the K to scroll down, not the J.
As a VIM User, I feel right at home :)
The navigation was obviously written by someone who prefers vim.
These appear to be based on VI key bindings (hjkl).
This seems to be influenced by tumblr, which also uses j/k. Tumblr is, of course, influenced penultimately by vi, which is ultimately influenced by ANSI control codes LF and VT :-)