Farm solutions, which are hosted in the IIS worker process (W3WP.exe), run code that can affect the whole farm. When you debug a SharePoint project whose Sandboxed Solution property is set to "farm solution," the system's IIS application pool recycles before SharePoint retracts or deploys the feature so as to release any files locked by the IIS worker process. Only the IIS application pool serving the SharePoint project's site URL is recycled.