I agree with @Walter Villa but I think the concept generated from horizontal three dots used for trimming exceeding text to control the flow of UI design representing there are more content for e.g.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce felis ligula, commodo vel nibh ac, dapibus dignissim nulla. Nunc lectus erat, euismod quis...
So I think this same concept used in this three vertical icons to represent more option.
Nowadays it is kind of standard icon for "more option" specially in android. There is no strict rule to use it for "more option", there is always freedom to use any other icons but as it is getting famous and users are used to know it for "more option" so it's better to use it.