Power BI Desktop has relatively high requiremetns on memory to perform its operaitons or do calculations. That's why we pose minimum requirements on installing Desktop Get Power BI Desktop - Power BI | Microsoft Docs. When you close Desktop, there maybe still some backend-end processing operations like saving report data so it's process will not be killed right away. Normally, you don't need to worry about it. If you really hope its process can be terminated immediately, you can end task in your tast manager.