Sunday’s explosion, which was first reported by The Washington Post, occurred before the engine was lit, a source told Ars. It happened during a procedure known as LOX drop, in which liquid oxygen is added to the engine to determine if there are any leaks. At that point, something caused the fluids within the rocket engine to ignite. Testing of the Block 5 Merlin engine will be suspended until the cause of that ignition is found and fixed.
https://arstechnica.com/science/2017/11/an-experimental-spacex-rocket-engine-has-exploded-in-texas/
Maybe a misplaced drop of oil or dab of grease somewhere.