Habitual behaviors are automatic responses to specific cues, acquired through repetition in stable contexts. While habits can support long-term behavior change, relatively little is known about which factors influence their development. We conducted a preregistered reanalysis of a six-month longitudinal study on habit formation during the COVID-19 pandemic. Participants (N = 254) completed monthly surveys assessing behavioral automaticity (i.e., habit) and ten potential determinants. Cluster analyses revealed two trajectories: habit formation and habit degradation, with cluster membership differing by behavioral complexity, frequency, and context stability. Multilevel models showed that six determinants were significantly associated with habit strength: autonomous motivation, behavior frequency, context stability, feeling energized, feeling pleased (positive), and perceived difficulty (negative). A significant interaction between time and behavior frequency emerged after correcting for multiple testing. Findings underscore the dynamic nature of habits and suggest that tailored interventions may be especially important during early formation and to prevent later degradation.