If your degas bottle is being pressurized you either have an injector cup leaking, head gasket is blown or holes in the cylinders from cavitation. If you already had 1 injector cup that was cracked there could be more ( did you replace them all?). The only way to pressurize the coolant system like that is to have combustion gasses getting into the coolant system.
One other possible theory for the knock, you could have hydro-locked a cylinder that the coolant leaks into. When the engine is not running the coolant will enter the cylinder and fill it up. When you go to start the motor it will pressure the coolant up too much and can bend a connecting rod. I hope that is not the case but definitely possible.