If it's an electric hot water heater, The elements might be pulling more power than the old one and tripping the breaker. Most water heaters have have two heating elements, one at the bottom of the tank and one at the top. These elements can run independently or both at the same time. So, when using small amounts of hot water, only one element would be on, but if a significant amount of hot water were used, both elements would be used and pull more amperage than the breaker is designed for. My recommendation is to try and reset the breaker and if it happens again, contact an electrician. They will be able to quickly see how much amperage is going to the water heater and upsize the breaker if need be.