NSX has a housekeeping job which checks if there are any bloated indexes and if it finds some, it will reindex those again.
This housekeeping task raises this System event when it finds any bloated index.(Event code - 400007, Severity - Critical)
Since this is a critical issue which could cause performance issue, event severity is critical.
Once the reindexing is done, it generates a successful event(Event Code - 400008, Severity - Informational).