In many situations, it is of interest for authentication systems to adapt to context (e.g., when the user's behavior differs from the previous behavior). Hence, during authentication events, it is common to use contextually available features to calculate an impersonation risk score. This paper proposes an explainability model that can be used for authentication decisions and, in particular, to explain the impersonation risks that arise during suspicious authentication events (e.g., at unusual times or locations). The model applies Shapley values to understand the context behind the risks. Through a case study on 30,000 real world authentication events, we show that risky and nonrisky authentication events can be grouped according to similar contextual features, which can explain the risk of impersonation differently and specifically for each authentication event. Hence, explainability models can effectively improve our understanding of impersonation risks. The risky authentication events can be classified according to attack types. The contextual explanations of the impersonation risk can help authentication policymakers and regulators who attempt to provide the right authentication mechanisms, to understand the suspiciousness of an authentication event and the attack type, and hence to choose the suitable authentication mechanism.