Funky water bottles can happen any time you put anything besides water into them. The equation for the perfect funky water bottle storm is something like this (+/- a few variables):
juice and or coffee + hot car + time = FUNK
In the image below we left this insulated Klean Kanteen in the car for a few days with tea in it. When we opened it up there was mold everywhere growing on not only the tea bag, but the walls of the water bottle themselves.