The difference between this and the quoted lyrics (brothættunni vs brothættan) are because Sigur Rós for some reason doesn't conjugate properly so the original lyrics don't make much sense in Icelandic.
I don't know who transcribed the lyrics, but this is a misunderstanding. Sigur Rós do write unusal lyrics, but they do not write bad grammar. The lyrics run as follows (some of the words are difficult to make out, but I think this is it):
en biðin gerir mig [leiðan] brot[hættan] ég fer að sparka frá mér [og kalla á] ég verð að fara [hjálp]
(The words in brackets are those sung by voices other than Jónsi's.)
This reasonably parses as follows:
En biðin gerir mig leiðan, brothættan. Ég fer að sparka frá mér (og kalla á hjálp). Ég verð að fara.
Rough translation: “But the wait gets me down, makes me fragile. I start to kick my feet (and cry for help). I need to leave.”
(The words in brackets form a separate sentence which, as sung, is cut in two by the sentence
ég verð að fara.)
EDIT: I should have explained the following ambiguity: Taken in isolation, the form
brothættan can be either the nominative singular of the noun
brothætta , “fragility” (the
-n being the suffixed definitive article), or the accusative singular masculine of the adjective
brothættur, “fragile”.