What you are seeing is caused by Scheduled Switcher.
To control the Scheduled Switcher:
- From Setup in Lightning Experience, enter Lightning in the Quick Find box, then select Lightning Experience Transition Assistant. From Setup in Salesforce Classic, click Get Started in the Lightning Experience Transition Assistant tile at the top of the menu.
- Select the Optimize phase.
- Click Motivate Users to Work in Lightning Experience to expand the stage.
- Turn onoff Switch users to Lightning Experience on a regular schedule.
- Choose the switching frequency.
- To disable automatic switching for a specific group of users, create a permission set using the Remain in Salesforce Classic user permission. See Create Permission Sets. Chatter Free and Chatter external users can’t be excluded from automatic switching using this permission. The ability to create a permission set for a small group of specific users isn’t available in the Group Edition.