Internal hackathons are widely used to drive innovation and collaboration in software companies, yet not all succeed. Many fall short due to low employee engagement. Why do some employees dive in, while others hold back? This article explores what drives and hinders engagement based on insights from five companies practicing regular hackathons. Using Social-Cognitive Theory, we show that engagement depends on two key beliefs: one’s ability to contribute (self-efficacy) and the value of participation (outcome expectations). These drivers are not arbitrary; they are predictable, and thus manageable. By understanding the psychology behind engagement, organizers can create hackathons that help more employees to feel confident and inspired to participate. We offer practical guidance to make your next hackathon truly engaging.