This normally occurs after the download is finished but it's unzipping and installing. This can happen to any game download and installation, not just Destiny 2. A lot of people tend to notice it for ARK because the files and usage space are pretty big even with the smaller updates. It can normally be resolved by pressing the Pause button, then closing the Steam program entirely; wait until it's fully closed (make sure you right click on the icon in your task bar and close it fully, not just the window). Once Steam is closed, run it again, and it should continue the installation as normal. If you continue to have an issue, you'll need to check Valve's Steam support to figure out if there's other ways to fix the issue, as the downloading and installation happens through Steam's installation systems.