In this case, it's because of 두음 법칙 (Initial sound rule), which forbids ㄹ and 냐/녀/뇨/뉴/니 at a word-initial position. ㄹ becomes ㄴ, and 냐/녀/뇨/뉴/니 becomes 야/여/요/유/이 when it's placed in front of a word. Of course, loanwords are an exception (e.g., English radio -> 라디오 (NOT 나디오), Japanese ニス nisu -> 니스 (NOT 이스)).
력(力) turns into 역 when it is the first syllable of a word, such as 역량 (력량 -> 녁량 -> 역량). But it does not when it is the second syllable of a word, such as 노력(勞力).
The reason why 두음 법칙 exists involves rather complicated historical linguistics, so I won't get into that here.
Generally, the reason why one Chinese character has multiple readings is because:
Because of Initial sound rule, as described above.
Because the character had multiple Chinese readings from the start:
For example, the character 樂 has 4 different readings in Korean: 악, 락, 낙, and 요. This is because the Chinese used the same character 樂 to write different words. The reading 악 is used when 樂 is used to mean "Music" (e.g. 음악(音樂) "music", 악보(樂譜) "sheet music"), 락(낙) is used when it is used to mean "to enjoy" (e.g. 낙원(樂園) "utopia", 쾌락(快樂) "pleasure"), and 요 is used when it is used to mean "to like" (e.g. 요산요수(樂山樂水) "to like nature").
Rarely because Korean people made new pronunciations for some of the characters:
串 has 4 different readings: 곶, 관, 천, and 찬. The last 3 readings are derived from Chinese, while the first one, 곶, is made up by Koreans to mean "cape".
金 is pronounced 금 when it means "metal, gold", and 김 when it's used as the Korean surname "Kim", or in Korean place names such as 김포(金浦) "Gimpo" and 김천(金泉) "Gimcheon". The latter pronunciation is made up by Koreans.