In Arizona, monsoon season generally runs during the summer months heading into September. The heaviest rainfall in Arizona usually occurs during the months of July or August. Therefore, there is a correlation between rainfall amount and the monsoon season in Arizona.

I think its because of the amount of heat or pressure on that area, the more the heat, the greater evaporation and leads to formation of heavier clouds and then build a heavy rain. I hope you are satisfied with my answer and feel free to ask for more if you have questions and further clarifications