One of the most well-known laws of software evolution suggests that code quality deteriorates over time. Following this law, recent empirical studies have brought evidence that Technical Debt (TD) Principal tends to increase (in absolute value) as the system grows, since more technical debt issues are added than resolved over time. To shed light into how technical debt accumulation occurs in practice, in this paper we explore specific maintenance activities (i.e., feature addition, bug fixing, and refactoring) and explore the balance between the technical debt that they introduce or resolve. To achieve this goal, we rely on studying Pull Requests (PR), which are the most established way to contribute code to an open-source project. A Pull Request is usually comprised by more than one commits, corresponding to a specific development / maintenance activity. In our study, we categorized Pull Requests, based on their labels, to find the effect that the different maintenance activities have on the accumulation of technical debt across evolution. In particular, we have analysed more than 13.5K pull requests (mined from 10 OSS projects), by calculating the TD Principal (calculated through SonarQube) before and after the Pull Requests. The results of the study suggested that several labels are used for tagging Pull Requests, out of which the most prev-alent ones are new features, bug fixing, and refactoring. The effect of these activities on TD Principal accumulation is statistically different, and: (a) the addition of features tends to in-crease TD Principal; (b) refactoring is having an almost consistent positive effect (reducing TD Principal); and (c) bug fixing activity has undecisive impact on TD Principal. These results are compared to existing studies, interpreted, and various useful implications for researchers and practitioners have been drawn.