why weak vanderwaals forces strong near the critical temperature of the gas?

Critical temperature (Tc): The maximum temperature at which a gas can be converted into a liquid by an increase in pressure. Since in liquid state the molecules/particles are nearer having stronger attraction forces as compared to gases. That's why weak van der Waals forces becomes strong near the critical temperature of the gas.
At a temperature greater than the critical temperature, the gas molecules are far apart from each other, they have high kinetic energy are can move freely , so they have very weak force of attraction between them.
Further the van der Wall forces becomes weaker as the temperature increase.