Inproper seating.

Start by looking for an area where the tire is a bit farther out than average, and deflate and walk the tire from high to low.

That said, I've seen tires leaning to undersized where you have to work hard to seat them evenly. Sometimes, a bit of soap water in the area, combined with high pressure, and you trying to lift the tire by rolling it to the other side solves it.

So you know it's neither you, or some new problem, they were making tools for this job for the last 40+ years.