Background: In order to support efforts to prevent bullying, more needs to be understood about students' own explanations of bullying in their everyday school lives. In-depth qualitative analysis can contribute important insights regarding insider perspectives in terms of how students understand and explain the social interaction patterns of bullying. Purpose: The aim of the study was to examine, in detail, how a small sample of lower secondary school students explain why bullying happens and to develop a grounded theory analysis based on the students' perspectives. Method: The participants in the study were 17 Swedish lower secondary school students aged between 13 and 15 years. A total of 17 qualitative interviews and 3 follow-up interviews were conducted. Grounded theory methods based on a constructivist position were used to explore and analyse the data. Findings: The findings are based on data collected from young people who had witnessed bullying. The analysis of their explanations of why bullying happens resulted in six categories: social positioning, victim constructing, bullying normalising, rule diffusion, rule resistance and cultural ideals. These categories are interrelated, and the core process of bullying was identified as social positioning. The analysis suggested that the main concern of those who engage in bullying is to gain and maintain a high social status. Victims, in turn, were socially constructed as 'different' and 'wrong', and were connected with a low-status position. Conclusions: The study draws attention to the need for students' understandings of bullying to be considered -- for example, through student consultations. It is hoped that the current findings could be helpful as a starting point when investigating students' perspectives and giving students a voice in bullying prevention approaches at school.