It was not intentional. There must be some mechanism to disconnect the external propellant lines before lift-off and they obviously disconnected unintentionally after the static fire. The best guess based on the way the flame spread was that both lines disconnected and there was a mixture of methane and oxygen just waiting for the right fuel-oxygen ratio before it was ignited on any number of post-burn hot spots. (I haven't seen any new info recently so this explanation may have been already superseded by more recent analysis.)
From just the video it didn't look like a very fast burn. I wouldn't be surprised if it was just methane, or at least a non-stoichiometric mixture. (Could also just be poorly mixed).
Scott Manley posted on Twitter [0] that it looks like it was a proper detonation (supersonic flame front, etc.) based on some math given the known size of the test rig and rocket.
