Rampant means wild or out of control. Unruly children might run rampant at the supermarket, knocking cereal boxes off shelves and thoroughly annoying the customers.
If you're running rampant, you're on a rampage. Both come from the French word ramper meaning "to climb, creep" like an animal on hind legs, paws in climbing-mode, or like wild plants such as the kudzu that ran rampant over an old barn until the entire thing was covered. There can also be rampant wildfires that destroy houses in the Southwest, or robots that run rampant in the lab after the janitor accidentally sets them free.