When experienceing spikes in delivery/premature delivery for campaigns, please check the following:
Does the goal have Frontload enabled?
If a goal is set to, for example, 30% frontload, it allows the campaign to overdeliver until reaching 30% of the goal.
Go to: Pulse - Settings - Inventory Forecast
Make sure you haven't checked the box "Override" and entered a manual value. That cause the campaigns to overdeliver.
Manual Inventory Forecast override should only be activated on new accounts that don't have as much inventory, in order to set an approximate inventory for the Forecasting tool. Other than that, it should never be activated.
If used, the manual inventory figure (the number represents inventory per week) should be as similar as possible to the real inventory figure.
Considerable increased inventory
If you see a difference in the amount of inventory for the latest period, for example you see spikes on the days when the issue was reported, that could be the cause of premature delivery.