I guess you accidentally pressed alt-gr or some other many-level shift key, which changed the - from a normal dash to some utf8 dash. Tar then interpreted it as a file name, and created an archive called "-jxf" containing the setxkbmap file.
Thanks for the input, but I don't think so.
I can't replicate it, but what happened back then is 'j' option specific.
With or without dash, doesn't matter.
tar zxf something.tar.gz --> O
tar jxf something.tar.bz2 --> X
Called from busybox directly was fine
busybox tar zxf something.tar.gz --> O
busybox tar jxf something.tar.bz2 --> O
A potential busybox bug ?