If your post above is indicative of what your file looks like (i.e. two pairs of [ and ]), then that may be your problem. Although that’s valid JSON it’s not a valid key binding file and makes Sublime generate messages like the following to the console every time you save (at least in the build of Sublime that I’m running here):
Unable to parse binding [{command: next_view, keys: [alt+pageup]}, {command: prev_view, keys: [alt+pagedown]}]
If that’s the case, remove the extraneous [ pairs and that may help. e.g.:
[
{ "keys": ["alt+pageup"], "command": "next_view" },
{ "keys": ["alt+pagedown"], "command": "prev_view" },
]
Note that the way that Sublime works, the defaults (from the left pane) are always used, and the user contents (in the right pane) are just “addendums” that augment the defaults. So you don’t want to duplicate the defaults into your user preferences, just add your own customized bindings to it such as the two here.