Why is a chemical equilibrium described as dynamic?

The equilibrium constant does not indicate whether a reaction is in static ordynamic equilibrium because it is simply the concentrations of the products divided by the concentrations of the reactants. A reaction is at dynamic equilibrium if the rate of the forward reaction is equal to the rate of the reverse reaction.

Because when equilibrium is reached, the reaction is still occurring in both directions, it's just that rate (forward) = rate (reverse) so there is no net change in the concentrations of the reactants or products.