Tire age

Modern tires are a complex blend of polymers. The ingredients that keep the rubber flexible and "sticky" are the ones that tend to dry out and disappear over time. That's why you start seeing fine cracks in the sidewalls, and why you lose traction. Eventually you wind up with something like the 50-year-old spare in my trunk (since replaced) — zero road miles, but it's as hard and shiny as a billiard ball. I think your tires are telling you "it's time".