What makes a roof do this?

Is this due to overheating? There was virtually zero ventilation in this attic until the seller recently put on a power vent.



It looks like a combination of a swelled sheathing seam and or a nail backing out (or combination)

Dips in roof are often nothing more than framing members improperly crowned although such condition is cosmetic only.

You really need to show us pictures of the underside framing in this area.
Also was the whole roof like this or only this section?
What was the ambient temperature compared to the attic temperature?
Any other type of venting then power?
How was the insulation installed?

There are so many possibilities.