They mostly get dehydrated through neglect and bad husbandry. If your snakes are eating, and having good sheds and are provided with clean water, they should not get dehydrated.

Stress could lead to dehydraton, but the snake would probably go off feed long before you noticed anything like you saw in the picture posted in the other thread.

Sickness can be another factor leading to dehydration, but that would most likely manifest itself by other symptoms first.