Broken cords? Perhaps.....
I checked and found that the back tire has a couple of flat/high spots that are maybe 1/8" from low to high.
But an un-seated tire makes high/low spots in pairs, opposite each other. The OP writes "a couple" & "high / low spots" which reads more likely an unseated tire.........unless of course the tie experience 2 areas of broken cords while being "not over inflated". Well, that's what I think anyway....plus you get to buy the cool tire tool in the process of diagnosing the ailment