At high pressures molecules are colliding more often. This allows repulsive forces between molecules to have a noticeable effect, making the molar volume of the real gas greater than the molar volume of the corresponding ideal gas, which causes $Z=\frac{P V}{R T}$ to exceed one. When strong repulsive forces among the molecules of the gas dominate, making $Z >1$. The closer the gas is to its critical point or its boiling point, the more $Z$ deviates from the ideal case.