Why does a new hot water heater cut off breaker?

The most likely culprit is the immersion element. After a few years these elements crack and water gets in. The current from the heating part of the element infiltrates to the outer jacket of the element. This causes a higher than normal current flow. This will continue to get worse and finally you will end up with a short circuit where the breaker will trip instantaneously when turned on. The good news is that these elements can be replaced. The element will unscrew out of the tank and the exact wattage replacement can be screwed in. Remember to use pipe dope on the treads when reinserting the new element other wise it will leak from the connection point.

Reconnect the electrical connections and it will be good to go.