Crowdfunding is changing how, why, and which research projects are pursued. With the increasing number of crowdfunded research projects, it is important to understand what drives scientists to launch crowdfunding campaigns and how it affects their work. To better understand this recent phenomenon, we present a grounded theory of how and why scientists crowdfund. Through 27 semi-structured interviews, we find that scientists are motivated to crowdfund in order to share their work and engage the public in the research process in ways traditional science work has not offered. Scientists also perceive crowdfunding as a more accessible way to get funds quickly compared to existing fundraising mechanisms, such as grant applications. However, they must learn to use more accessible language to successfully communicate their research through social media to a broad audience of non-scientists and professional peers. Based on these findings, we discuss design implications to inform future crowdfunding platforms and support tools.