First, it may be because the tutor received a new topic or project during the winter and summer vacations. At this time, graduate students need to devote themselves to new topics.
Second, if there are no topics and projects, most graduate students will choose to conquer courses or complete graduation thesis. It often takes a year or so for graduate students to complete their graduation thesis from preparation to research.
In addition, graduate students need to spend a lot of time preparing for internships and exams.