The effectiveness of collaboration is often explained by the alignment of social networks with collective-action problem characteristics, yet previous research on social tie formation has focused almost exclusively on actor and relational attributes. We theorize that collective-action problem characteristics together with actor and relational attributes explain social tie formation and that the relative effect of these factors varies with uncertainty about collaboration partners. The study tests seven hypotheses associated with these factors by estimating multilevel network models of collaboration and task engagement among managers responding to a major wildfire in Sweden. The combination of actors and tasks in a single model captured key characteristics of the collective action problem (task engagements and task interdependencies), and disentangled the relative effects of these factors from actor and relational attributes. Results suggest that social tie formation can be explained both by actors' task engagements, and actor attributes associated with leadership, professionalization, and experience. Further, the effect of task engagements decreases in organizational relationships where collaborative uncertainty is high. Since the alignment of social ties with problem characteristics is supposedly positively associated with collaborative effectiveness, this finding suggests that risk-aversion is a more deep-rooted driver of tie formation than the pursuit of collective performance.