Recently Nonprofit organizations (NPOs) are adopting more and more data-driven approaches to their work, yet NPOs often lack appropriate tools and expertise in such data related works. To compensate, many NPOs are using a new form of collaboration, civic data hackathons, to leverage on external volunteers' data expertise. In this paper, we sought to understand how civic data hackathons could generate impactful data analytics for NPOs' data-driven work, and how to support collaborative data analytics during hackathons. We collected various types of data (observations, surveys, and interviews) from two civic data hackathons with 9 NPOs and over 300 data volunteers in a Midwestern city in the U.S. Our results describe the collaboration activities and the types of actionable collaborative analytics outputs generated from these activities. We also identify a unique social group (i.e., client teams), who help with preparing and coordinating the event, perform brokering activities to support the collaborative analytics through the civic data hackathons. This broker role is vital for the success of the collaboration between domain experts and data experts. Our findings contribute to the CSCW research on the collaborative work of interdisciplinary hackathons, and to a broader understanding of civic data collaborations.