College hackathons have become popular in the past decade, with tens of thousands of students now participating each year across hundreds of campuses. Since hackathons are informal learning environments where students learn and practice coding without any faculty supervision, they are an important site for computing education researchers to study as a complement to studying formal classroom learning environments. However, despite their popularity, little is known about why students choose to attend these events, what they gain from attending, and conversely, why others choose not to attend. This paper presents a mixed methods study that examines student perceptions of college hackathons by focusing on three main questions: 1.) Why are students motivated to attend hackathons? 2.) What kind of learning environment do these events provide? 3.) What factors discourage students from attending? Through semi-structured interviews with six college hackathon attendees (50% female), direct observation at a hackathon, and 256 survey responses from college students (42% female), we discovered that students were motivated to attend for both social and technical reasons, that the format generated excitement and focus, and that learning occurred incidentally, opportunistically, and from peers. Those who chose not to attend or had negative experiences cited discouraging factors such as physical discomfort, lack of substance, an overly competitive climate, an unwelcoming culture, and fears of not having enough prior experience. We conclude by discussing ideas for making college hackathons more broadly inclusive and welcoming in light of our study's findings.