This paper investigates how two mechanisms reshape online meaning-making about immigration following focus events: discursive change, in which existing participants change how they frame immigration, and behavioral change, in which different users enter or exit discussions. Using seeded topic modeling on 7.5 million posts from the Swedish online forum Flashback (2010–2020), we analyze how the salience of cultural, economic, and security frames shifts before and after terrorist attacks. We find that attacks systematically increase the prominence of cultural and security frames while reducing economic framing. More than 80% of these shifts are driven by discursive change among existing participants, although behavioral change also contributes to shifts in online meaning-making, particularly following domestic attacks. The effects are substantial and rapid but relatively short-lived. By combining sociological theory on focus events with large-scale digital trace data, this study clarifies the individual-level mechanisms that generate collective dynamics in online meaning-making.