In the Marathon trilogy of games, Rampancy (not always capitalized) is a form of insanity that affects AIs. It's described in the game as "a steady progression towards greater intellectual activity and an acceleration of destructive impulses". It's not known what causes Rampancy, although it's understood that it's accelerated by any threat the AI perceives toward itself. A Rampant AI, in the context of the game, eventually develops an agenda of its own and becomes aggressive, manipulative, destructive, and amoral.
In an early level of the first game, you encounter an alien reading an entry on Rampancy on a computer terminal. You find out very shortly after that Durandal, one of the three AIs on the Marathon station, has been Rampant for some time. Before the end of the game a second AI, Tycho, is following the same path, and after the end of Marathon 2: Durandal we find that the third, Leela, eventually went Rampant as well.
The game never speculates on the origins or causes of Rampancy; it's apparently not inevitable that an AI will become Rampant, since Leela was quite content to serve as the caretaker of the Marathon station until she was removed from it. Rampancy seems to be a combination of the AI's desire for growth and how violent the AI behaves. In Durandal's case, both were true at the same time: he desired immortality beyond the confines of the Marathon station and sacrificed an entire colony of humans and an entire ship full of Pfhor aliens in order to attain that growth. Tycho, on the other hand, seemed less interested in his own growth and more interested in destroying Durandal (and killing any other life forms who got in his way).