Why would a Teledyne Laars Series 1 natural gas 400k BTU pool heater 'explode' when turned on then after running about 15 seconds it will 'explode' again?

Most likely one or two of the burner orifices near the pilot light are clogged making the gas travel a distance before lighting. Pull the burner tray and clean it completely. And check for spider webs and other debris in the burners. Check for other gas leaks, and don't use flex lines, flanges, valves, or other fittings inside the heater. On the Series One, check the sidewalls to make sure they are intact.