Why is 因为 listed as an adverb in the dictionary?
All other dictionaries I checked do not list 因為 as an adverb, and I personally cannot use it as one. My guess is the source you quoted misprinted (副) adverb in that entry.
Besides 'conjunction', 'preposition' is the other usage  listed in Wiki,

owing to; due to; as a result of; because of

I can come up with examples with that:
因為 as preposition:

他因為貧窮而賣子女 (he sold his own children due to poverty)

他因為錢財糾紛而殺人 (he killed people because of money disputes)


因為 as conjunction:

sentence 1: 他賣了兒子給同鄉的富戶 (He sold his son to a wealthy family in the same town)

sentence 2: 他實在養不起十個孩子 (He really can't afford ten children)

with conjunction: 他賣了兒子給同鄉的富戶(是)因為他實在養不起十個孩子