[G Larry]

My guess:

Oxidizer build up in the flame bucket pre-ignition, then when fuel is added at ignition, we get this result.

[pfflier]

Sounds plausible. LOX coming in contact with any kind of hydrocarbon can do this. The catalyst can be an ignition source or even pressure.

The event appeared to be external of the booster, like you speculated, near the base ring or the flame bucket. Regardless, that booster is definitely going back to the shop for some rear end repairs.

[markman46]

it was a mixture of liquid o2 and methane they were doing a spin test of the engines and boom nice flame and scorched orbital launch mount legs. took spacex a long time to detank the booster, happened at 4:20 cdt and didn’t finish detanking until oh 10:00 ish cdt.