The nails slip out of the sheathing and create small visible bumps on the surface of your roof.
Roof plywood buckling.
Buckling can occur if the wrong type of nails were used to connect the sheathing to the rafters.
Wood expands and contracts so there may not have been adequate planned spacing.
Improper installation of roof sheathing panels can result in warping or buckling of asphalt shingles.
Roofing problems can be tricky to diagnose and solve but the solution here should be pretty straightforward.
Most underlayments have an adhesive side and a barrier side.
Buckling roof plywood below asphalt shingles is a result of many things.
But they must be installed correctly.
I agree that it looks as if the buckling is at sheathing butt joints at rafters.
Your roof decking is the plywood sheets that cover your home.
If the adhesive side is not laid properly and not applied flat it will surely show its buckling through the roof shingles.
For a good reason.
Check the buckling pattern for spacing at 4 or 8 ft intervals or a multiple of that.
A roof over or re roof is not recommended because it will certainly lead to buckling.