Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means detach, but what happens when something detaches? It causes it to separate and to leave. This refers to when something separates on its own. You aren't doing the separating. You know this because it's rare (れる) to see things separate, but sometimes you do see them. Also, whenever you see れる usually it's a safe bet that you're not the one doing the action, right? Think about it.