Answer:
Monsoon season.
Step-by-step explanation:
The monsoon season normally occurs in Bangladesh from June to October. This will cause tempurates to drop about 1/3 of the country will be really wet by the end of the monsoon season and it's 80% of the country so it makes it the most rainfall season in Bangladesh.
(PSA: I don't know if it's monsoon, but I hope it is since it is a season which cold winds travel to a country and causes the county to be really cold and wet)