This kind of event occurs in two cases:
1. The start date of an activity (of a given type) has been changed to the current date manually, or
2. The start date of an activity is changed to the current date automatically as a result of a dependency, e.g. Finish to Start. (example: master activity's Actual completion date is today, lag = -1, so dependent activity's Start date changes to today).
NOTE: The Start notifications are NOT sent for activities that are on hold or closed.