Brand new hot water heater installed and blew breaker in less than a week! Its only happened once so far but curious as to why it would blow it!

Short, overload or a bad breaker. Can you tell us the brand/model and also info about your breaker. What was the old heater doing when you decided to swap it out?

Is the new heater bigger? What wattage elements were in the old one? And the new one?
What size wire feeds it and what size breaker?

Same hot water heater as old one, just new! I installed it for someone else, hooked it up exactly the same as it was. I'm assuming maybe the elements aren't coinciding and kicking it maybe! 30 amp breaker, wired direct.

Well, with a new unit you should have not had to go inside of it. The ONLY wiring you need to access is the main line in at the top. There is really no way to wire it wrong other than creating a direct short, which would trip immediately.

Are you sure all the specs were the same? Wattage and voltage?

An Amprobe would save a lot of time here.
Like Stan is saying, only the connections at the top need to be dealt with.