Triggered Send status Inactive or Paused

There could be a few possibilities where the triggered send on an active journey could be set to 'Inactive' status without the User's observation apart from intentionally pausing the journey or the triggered send from Email Studio or an error that stopped the triggered send with an Automated notification. 

1. There is a known issue when after Editing the email activity on the journey canvas and Saving it, then clicking it back again and navigated away, the system cannot Save the Triggered send which leaves it in 'Paused' state.

https://issues.salesforce.com/issue/a028c00000gAxJaAAK/navigating-away-from-journey-email-activity-saving-window-will-cause-failures

2. It is possible that there are multiple tabs open that might have caused caching issues which would not activate the journey automatically and that requires a manual intervention.

3. One more possibility is that once the journey was activated and another User clicked the Email activity not knowing it would cause emails to get paused or accessing the journey at the same time.

The most likely issues that were observed were the three above which potentially require a manual intervention. These were the findings based on past research and experience with Journeys, triggered sends and email activities.