How do bouncing microwaves inside the oven cause hot spots?

Firstly, temperature is defined by the amount of vibrations occurring in a material's atoms. Certain materials, like water; fats and sugars, can be polar molecules (which means due to certain characteristics, they have a negative and positive end). Microwave ovens create an electrical field of electromagnetic microwaves, which attract the polar molecules into a certain direction.

Because microwaves oscillate (bob up and down), the polar molecules are therefore pulled up and down with them, causing them to vibrate. This vibration, as explained above, causes the temperature to increase.