@defect clustering
And in addition, in my experience, there might be factors outside the development team that will also cause defect clustering;
*scope creep was not addressed (less time for large amount of functionality, vital changes late in the process, changes in other parts of the system)
*ambiguous descriptions - badly described functionality - misunderstood use cases
Defect clustering is also proven in research, so it is not only something we as testers can think of as a “truth”.