「北」, in the word 「敗北」 is derived from, or is in any case related to, the kanji 「背」. If you look 「ほく」 up in the dictionary you will also find 「背をむけること。にげること。」 (to turn one's back, to flee)
「敗北」 doesn't just mean "to lose" but also has a connotation of "to give up" or "to flee".