All incident SLA notifications are sent at the breach time

All incident SLA notifications(50%, 75% & 100%) are sent at the breach time. 

In the SLA definition, the breach time is set as as 'Due date' and in the Start conditions: Retro active start > 'set start to' is set as 'due date'.

That's why the SLA starts when it matches the due date.

Change Retro active start --> 'set start to' = created

OR

Uncheck the "Retro active start" field.