It is reasonably possible that the wheel itself had some damage from use with the standard tyres, that you were totally unaware of (inside crack or something similar) and then when you pumped up using the tubeless the damage was a ‘weak spot’ that led to the failure.
The inner tube taking the majority of the pressure previously may have been why the wheel didn’t fail until now; at which point having the ‘direct’ pressure was unfortunately enough to create the failure.
Perhaps you were fortunate it happened now while just pressurising the tyres rather than at 20mp on a downhill.
Likely to be something you would have had no chance of spotting visually beforehand with an inspection so will be hard to identify what the actual ‘cause’ was. Do you recall ever having a back ‘knock’ on that wheel at any point?
I think the manufacturer may well take that stance as you have had the wheels a fair while now, but no harm in asking them to see what they say.