How about moisture damaged sheathing due to ice damming?
We already have that one Randy.
Built in 1974. Two story apartment buildings.
Trussed roof rafters. 
That crossed my mind also Randy.
This is how I got on the roof. Did not trust the staight ladder 2/3 of the way up.
Once on the so, so, deck, I pulled my ladder up and went on the roof.
The hump was on the other side. 
Ok my new guess is that the shingles were installed to close together and from the heat they expanded and buckled.
Good guess Greg, I thought that too but then why just in a two foot section?
Another hint.
Replaced some wind remove shingles not very well done .
I wondered that as well but kinda grasping at straws. I have installed a few shingles in my life but never had this issue. It is almost like there was something on the roof and they shingled over it. Thats why I said a vent.
Ok now that I see you picture, they shingled over a prybar???
That is funny.
This thing bugged me for a week after I saw it.
Was directed by owner to investigate the cause.
I felt like I was doing a C-Section for Krist sake, but then I was more worried about falling of the edge of the roof.
Well, the ice and water shield is about to burst at the seams.
What is it?
Looks Like a piece of head flashing nailed to the roof for reasons I cannot comprehend. Must be a Winslow thing. LOL
Heat blister who would have thought about that . Thanks Marcel
Nope!
Well, at that point I knew what it was, so had to go in the attic to see how the hell it happened and what caused it.
Luckily, when I came down the roof, I discovered that the apartment was vacant and the door off the deck was unlock.
So I let myself in.
And grab my nose and make sure the door stays open.
Wow!
Alright, found the attic access.
Hey, they even used nails to install it. How about that?
Time to go up and find out what is going on.
That is not it.
Well, that is not it, but at least we know it is 1/2" plywood. Cheap bastards.
Not even H-clips with that thin chit.
Ahh, there it is, look at that, edge of the sheet curled right up 1-1/2".
Now what would have caused that?
The sheathing was installed to tight!!
The joint is on the right side truss. Looks like new Sheathing.
Nope! If that were the case, the sheet would have buckled in the middle.