Shareholder activism is commonly seen as a tool used by shareholders for attempting to ultimately increase market capitalisation of underperforming firms. However, empirical evidence is inconclusive on the effects of shareholder activism on market capitalisation, measured by abnormal returns. In this paper, the issue of what explains the occurrence of shareholder activism is analysed. With the aid of two illustrative case studies of shareholder activism, this paper elaborates the notion that shareholder activism is not always an offensive seizure of an opportunity to increase performance and thus market capitalisation; it may sometimes be a defensive reaction to increasing costs for using the exit mechanism. The latter type of shareholder activism need not be associated with abnormal returns. An implication of this is that instruments used in empirical studies of shareholder activism needs refinement in order to differentiate between these types of shareholder activism, especially in order to make valid assessments of its efficiency.