Multi-agent randomized behavior
Someone who owns (an abstract of) the book might be able to give you a better contextualized answer, but in general models and simulations are subject to certain 'laws' that the systems they represent usually are not. In the case of multi-agent systems, one of these is the fact that some agent gets to act first on a given time step, even though the designer often assumes and intends for them to act in parallel (this is the result of computers being incapable of truly parallel computation). If the same agent were to consistently act first, that could introduce significant bias into the system. This is probably what the author means with the pitfall of predictability and why randomization is important; so that we can prevent (to some extent) these sorts of issues.