What is it about stimulants that cause dehydration?

They can,but unlikey: Stimulants are what they say, they stimulate the body. This increases the amount of water used by the body. However the brain will recognize this and request more fluid intake. So unless you are doing heavy exercise and not maintaining proper hydration, there is not significant concern.

Depends on situation: Caffeine and nicotine both can have an effect on the water balancing hormone of the kidney and lead to extra water loss. In most cases it is not an important amount & the body accommodates the loss with more thirst. Other stimulants have variable or no effect on the bodies water loss.