Warm air has fast-moving molecules. The force of their impact upon collision is great, but when they collide, the power of the impact sends them shooting far away, expanding the atmosphere as they go.
As the atmosphere expands, it is less likely that these randomly moving molecules will hit each other. They have more room in which to roam uninterrupted. The frequency of impact is more important than the force of impact.
Low pressure, therefore, results from hot air: molecules moving quickly, but not colliding frequently in the expanding atmosphere.
Fast-moving, high-impact, infrequently colliding molecules = warm, low air pressure.