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Suppose the concentration of $NO_{2}$ is increased. There are more $NO_{2}$ particles per unit volume, and more collisions between more$NO_{2}$ particles occur. A higher frequency of successful collisions occur, and the forward rate is initially higher, as the reverse rate is initially unchanged, since there is no initial change in product concentration. More products are produced than consumed, so the product concentrations rise and so the reverse reaction rate increases. More reactants are consumed than produced, so the reactant concentrations fall and so the forward reaction rate decreases. This occurs until the rates are equal, and equilibrium is re-established having shifted right.
