purge rocket engines is common and has been used for decades. The nitrogen displaces gases and/or liquids, effectively cleaning the engine and preventing any volatile mixtures before ignition.
A check valve that allows the nitrogen purge prior to ignition in the Merlin engine was stuck open just before launch. This stuck valve allowed “liquid oxygen to flow from the main injector [for the rocket engine itself] into the gas generator injector” that generates hot turbine gas, which drives the turbopumps, according to SpaceX. The turbopumps are basically very high-powered fuel and oxidizer pumps that deliver the liquids to the main combustion chamber of the rocket. The result was the turbopumps were operating at a slightly higher power level, resulting in the high pressure detected in the combustion chamber on engine five.
