You may see auto-rejected orders if your Deliveroo browser has been logged out due to inactivity. Usually, this happens after 45 minutes.
You can download an auto-refresh extension for your browser if you find yourself regularly being logged out of your Deliveroo web browser.
Activating tabletless support for Deliveroo will prevent you from having to use the Deliveroo tablet or website, therefore avoiding this issue in the future. Take a look at this article for details.