Look at "背" carefully, its upper part is 北 with its lower part as "月". Here "北" contains the meaning as well as pronunciation. "月" is added to mean the "the back of a person". So overall "背" now has two major meanings, one is "Back of a person or something", second is "Go against".
Evolution history of "背" would show a clearer idea: (image get from http://www.vividict.com )
|Learn one Chinese Character a day – 背|
Now it is time to enjoy a Chinese calligraphy with 背 inside: