Some learning management systems have a quiz setting that causes this behavior. In Blackboard, the setting is called "Forced Completion".   Other learning management systems may have similar quiz settings.

If there is ever an interruption in the internet connection during the exam, these "Forced Completion" settings automatically submit the exam. This same behavior can occur if the student clicks on the "Refresh" icon.

In light of this, we do not recommend that these settings be used.