Online communication offers the potential for bridging connections, exposing users to new views and experiences by fostering socially heterogenous communities. However, in the absence of deliberate attempts to promote diversity, communities may tend towards insularity: a state where members and content are similar or homogenous, and where deviation from these norms is discouraged. This paper presents a taxonomy of processes contributing to insularity, synthesizing findings from a broader longitudinal interview study on engagement with online communities over time with previous literature. Using thematic analysis, sixteen processes were identified which were associated with four broad stages: formation (selective connections, network homophily, shared interests, audience segmentation); propagation (circlejerking, upholding community standards, avoiding conflict, tailoring content); reaction (individual avoidance, collective reaction, mocking deviance, derogating outsiders); and perpetuation (modelling, prior feedback, echo chambers, gatekeeping). These findings highlight the need to consider more diverse mechanisms by which communities become insular, and the role that platform features play in facilitating these processes.