It is not overflowing. The temperature/pressure relief valve is opening and letting water out. It is possible that the valve is faulty, but since it’s a brand-new heater, it is more likely you have a pressure issue. There is information on what this is about and how to test for it and fix it in my Tanklets section. Go to the homepage and click on Troubleshooting, then Tanklets, then Temperature-Pressure Relief Valves.
As to why the old heater didn’t do it, it’s probably because the T&P valve was clogged up and couldn’t open at all.
Both of the Lowe’s people’s scenarios are possible. The reducer could be the cause and the expansion tank could be the fix. Whatever, you don’t want to just walk away from it. Aside the the water running from the T&P, excessive pressure can wreck water heaters, appliances and piping.