Hot water can cause parts to become deformed. Just a small change in shape can affect juicing performance, cause leaking, or even prevent assembly of the juicer. Hot or especially boiling water can damage the magnetic sensors in vertical juicers, which means the juicer will think the unit isn't assembled safely and will not turn on. Even the seals can be damaged, which can potentially lead to juice leaking into the motor.
The rule of thumb is to never wash a juicer or its parts in water hotter than 80 degrees celsius. And don't dry parts in an oven or on a heater.