In a molecular model of a gas, the particles are in constant random motion, colliding with each other and with the walls of the container. When two gas particles collide, they change direction as a result of their relative velocities and the nature of their interaction. Similarly, when a gas particle collides with the wall of the container, it changes direction and continues on in a new direction.
The collisions between gas particles and between particles and walls cause the random, chaotic motion of the gas particles and result in the observed macroscopic properties of gases, such as pressure, temperature, and volume. The constant, random nature of the collisions is what causes the gas particles to change direction in the model