The kanji means riot, but this verb version word doesn't really mean "to riot." It's more like to be in disorder. But if you think about it, when things are rioting and crazy, they definitely are in disorder, so you can think of it that way. You can differentiate this from the future word 乱す because with れる you know it is rare for it to be like this. You aren't doing it, but it just happens on occasion (rarely) so you take advantage of it.