Although it seems that it can be better documented, it probably has something to do with it:
push.default
Defines the action to be taken git pushif refspec is not specified on the command line, refspec is not specified remotely, and no refspec option is implied by any of the options specified on the command line.
Possible values:
nothing - .matching - . . .tracking - .current - .
push refspec push.default, , - , , push refspec. , , , - -, push refspec.