This is because you have multiple documents open in separate tabs and there has been no activity (mouse and keyboard) in at least one of these documents in the last 30 minutes.
If you are actively using one document and not others then you should manually close all other documents to prevent this from happening. The alternative is to open those documents in separate browsers.
Note the following:
The Web Viewer automatically logs you out of the dashboard and closes documents after a certain period of inactivity.
You will automatically be logged out from the Web Viewer dashboard if there is no user activity for 1 hour.
An opened document will be closed after 30 minutes of inactivity (i.e. no mouse or keyboard activity). The document will be unloaded from memory and you will be returned to the Web Viewer dashboard.
If you have multiple documents open in separate tabs then if one of those documents is inactive for 30 minutes all your opened documents will close and you will be returned to the Web Viewer dashboard.