Forms such as concussed or discusses may lead people to wrongly double the final consonant of focus; yet, I cannot come up with a potential explanation of why people wrongly write biassed or the plural biasses.
Neither focussed nor biassed is wrong. They're less common alternative spellings. Likewise benefited and benefitted. There is also a BrE/AmE difference with several other consonants when the final syllable is unstressed; these mostly follow a simple rule, but the general question of final consonants after unstressed vowels has no single simple rule.