Running bamboo is notoriously fast spreading and difficult to remove. What keeps its population balanced in the wild, and prevents it from crowding out the competition? I tried googling, but was inundated with gardening advice, horror stories, and assault / offensive gardening (some of the latter two presumably covering the same incident from both sides). My google-fu failed, I couldn’t really find any info about natural population controls of running bamboo in the thicket of tall tales and gardening advice.
It is the same as other easily spreading plants. In their native habitats there are checks like diseases and predators. When you move them out of those habitats they can thrive at a new level because of the lack of those things.