It's grooved for better traction in the rain, helps with hydroplaning.
Most tires are affected by it when dry with the "wander" effect.
First time I felt it was in 1981 when I got my first new car, a Chevette and it was all over the road.
It has happened to me in every car/truck I have ever driven. I have even experienced a worse "wandering" effect on drawbridge steel open decks...