These things are rarely random. Probably crack propagation starting at the underside (where the tube is in tension) at the stress raiser caused by the edge of the lug
+1
The underside of the failiure looks to be right on the lug join. Also the butting of the tubing could very likely be around that area.
+1
The underside of the failiure looks to be right on the lug join. Also the butting of the tubing could very likely be around that area.
Guessing though.
Still a crying shame, whatever the cause.