Some have soil binding and seed suppressant abilities, like Tenacity. Others are simply toxic to seeds or young plants and they (or their metabolites) are capable of killing baby plants for longer periods of time, even if the herbicide can't impact an adult grass.

Metabolites can be more toxic than the herbicide and can have a longer lifespan, or less and shorter. Most are the result of bacterial action, although you'll also find those that break down by reacting with water.

It's very easy to "design" a chemically stable and long-lasting herbicide with excellent seed suppressant characteristics--we've been using it since at least Babylon and simply call it "salting the earth."