my guess (and I'm not a metallurgist, not even on TV) is this was just "bad luck" due to the tool being thin-walled, cast steel (and prone to having some hidden defects in that casting process) then heat-treated to make the splines "harder" but the entire tool more brittle, which is a trade-off.
If this had been made by forging and then finished machining it may not have "exploded" but probably cost a great deal more and there are still other ways a tool can fail (broke my heart when a beloved VERY thin-walled Phil splined FW tool "collasped" on me! But my replacement, same tool, has held up).