This study investigates the use of causal narratives in public social media communications by U.S. public agencies over the first fifteen months of the COVID-19 pandemic. We extract causal narratives in the form of cause/effect pairs from official communications, analyzing the resulting semantic network to understand the structure and dependencies among concepts within agency discourse and the evolution of that discourse over time. We show that although the semantic network of causally-linked claims is complex and dynamic, there is considerable consistency across agencies in their causal assertions. We also show that the position of concepts within the structure of causal discourse has a significant impact on message retransmission net of controls, an important engagement outcome.