What's interesting is that updated info indicates it happened before the engine (which was a block 5 engine, as you say) was ignited, when they do a "LOx drop" test, basically running liquid oxygen through it to test for any leaks:
https://arstechnica.com/science/2017/11/an-experimental-spac...
It may mean they had a simple FOD (foreign object debris) or contamination issue, not a design problem. Powerful oxidizers like liquid oxygen can make contact explosives out of all kinds of organic materials.
Stuff that you normally don't think of as flammable -- like hunks of structural metal -- burn quite nicely in the presence of liquid oxygen. So stuff that burns well, burns REALLY WELL in the presence of LOX.
It may mean they had a simple FOD (foreign object debris) or contamination issue, not a design problem. Powerful oxidizers like liquid oxygen can make contact explosives out of all kinds of organic materials.