Need some help understanding what happened here. Large area of shingles appear to have moved. What would cause this? At first look I thought it may be a repaired area but shingles are the same and have not been replaced. I thought maybe it might be from the roof decking (plywood) warping or moving but I did not see any evidence of that in the attic. No leaks, decking was in good shape. I have seen shingles come loose but these were single shingle, not anything like this.Any ideas what would cause this?
Looks like a bad installation to me .
Installed (poorly) using the “racking” method.
Dom.
I agree with Dom, poor installation, racking method of shingle install is inferior IMHO, even though not specifically prohibited by some shingle mfg.
For your report I would use this narrative.
“Asphalt shingle roofing appears to have been installed by cretins, drug addled morons or Trump administration cabinet members.”
Definitely an installation problem. First off installation using the racking method. The bigger problem is the movement. If you look at the lowest course of shingles that are misaligned, you can see that they are clawed (i.e., the tabs bunched up). This tells us that they we not originally positioned where they are now. They have moved. When you see a large section of the roof sloughing down that way, it is very likely to be due to the nails being over driven with an air gun. My money says that the roofers drove the nail heads completely through the shingles and there is nothing holding them in place but gravity and friction (gravity is overcoming friction at this point). It’s very possible that they did this across the entire roof, which would make it a full tear off. I’ve seen this before, but never seen such a large section slip so evenly.
That’s a really major defect. Not a good day for the seller.
This one was brand new and had to be completely torn off and redone.
https://www.nachi.org/forum/attachment.php?attachmentid=182596&stc=1&d=1488588513
https://www.nachi.org/forum/attachment.php?attachmentid=182597&stc=1&d=1488588539
Thanks everyone for the help in explaining what is going on here. Seams to a much bigger problem than I originally thought.
That condition is almost exclusively related to substandard shingle nailing, either high nailing or overdriven nailing is usually the cause.
Yep, Racking method improper for those shingles and gun pressure set too high. On the other hand, the fact that a bunch came off together may indicate that they bonded decently. 
I agree, probably a total re-roof.
on the brite side the tear off is going to be a snap…
What is the Racking Method that is referred to in this post?
It is when shingles are installed in a straight up application instead of side to side,
A home inspector making judgement calls on Trump in a forum- I thought the inspection business was less populated with snowflakes … ha
The roof shingles should always be dried on cold setting in the dryer or they will shrink.