A beast going berserk was quite rare as even though all of them had the option to do so many never resorted to it. This decision was the beast's alone to make; however, there were ways to exploit that method and manipulate the beast to forcefully go berserk.
The Beetle King of a colony had complete control over all those that were part of its colony and it used its control to order them around and do work or forcefully make them go berserk and rush Iris so that it could save its own skin.
The Terra Beetles were mindless soldiers ready to obey the will of their king by any means necessary. Due to being weakened by the team's strongest attacks from the very beginning, the Beetle King was already fighting Iris and Bill while being injured and weak, plus it was stuck in its Earth ability mode which was only level 5 and much weaker than its Level 7 Lightning ability because of Iris' continuous strong water attacks.
