Rumbles have become a tradition at most robot events here. The first one I can remember seeing was during an early ROBO-ONE competition in 2004. The concept is simple. Usually all of the robots start off in the ring and try to push, shove, or kick their opponents out of the ring without falling out themselves. Sometimes, when there are a large number of robots, their entry into the ring can be staged.
For a highly produced event, like Robot Pro Wrestling Dekinnoka 10! there may even be script or storyboard. Rumbles, either free form or scripted, are hilariously funny.
One of the most over-the-top rumbles I've ever seen took place during the summer of 2006 in Osaka. It started in the late afternoon and continued on well into the evening with the robot builders taking frequent breaks from the dinner table to stage another robot rumble.