Why not du må bytte passorder?
'passord' is a compound word: pass+ord. 'ord' is a neuter word, and a lot of monosyllabic neuter words doesn't have the '-er' in their indefinite plural:
et passord - passordet - passord - passordene
A lot, not each?
Passwords - passord / why not passordene?
"Passordene" is the definite plural, "the passwords".