Normally I would give you a vast explanation about how and why seasons shift. But I cant take the help of pics because musing still doesn't allow photos to be used in answer and without them it is hard to present. So I will keep my answer brief.
What triggers monsoon rain?
Monsoon is a season that you will elaborately notice in parts of asian country.. Other countries of the world has 2,4,5 seasons. But asian countries because of their global position and largest land mass face six diverse seasons. So when summer is at the end of its annual span, earth faces its first rotating shift for the year. The equatorial line gradually moves far from the sun. And countries near equator gets cooler because of wind revearsal.. But sun rays still fall almost perpendicularly causing a low pressure zone.. The vapour saturated sea wind comes to fill the void. the saturated vapour then when sun rays fall directly, heats up and forms dense rain heavy clouds.
Now when the sun shifts more the land mass gets clolder making temperature drop in wind. this makes the clouds and water vapor condensate and store heat. The more the condensate the more temperature drops and at a certain point clouds become condense enough to rain down on the land.
This process constantly keeps happening around the whole monsoon season.. And when earth reaches the far end of its orbit then season rotates to winter.