Lots of chemicals
The materials used to make your sneakers, especially rubber, have strong odors because of the chemicals as well as heating/proccessing needed to mold them into shapes.
Nontoxic materials actually often smell worse than toxic ones--have you ever smelled the 'skunk smell?" or rotten egges? They're nontoxic, but aweful! lead/merury i.e. Often have no odor but they are toxic. Have fun learning..