Events can be duplicated within a campaign and become "Orphaned" events which ACS recognizes as being at the "root level" of the campaign and thus executes all events at the same time. These "Orphaned" events may become stacked on top of each other therefore not visually noticeable. If users are loaded into a campaign with orphaned events, those events are triggered simultaneously
Inspect a campaign prior to loading users to verify no orphaned events exist. Sometimes multiple orphaned events can be stacked on top of each other. If you encounter any, delete those prior to using the campaign.
This bug can manifest itself when editing a campaign while multiple Campaign Studio tabs are open, or after copying a campaign.