Sounds to me like a one-off failure caused by the bush starting to rotate in the rod. Wonder what sort of fit the gudgeon pin was when it was put together? Not sure with these engines whether, per book, the pin 'should' swivel on the piston, in the eye, or both. Personally, I like them to be a close sliding fit in the little end, and a tight one in the pistons when cold (in my world, that's where a rag soaked in boiling water wrapped round the piston for a couple of mins is enough to allow the pin to move without the application of undue force). But there be varying opinions on the subject, I know.
Were it me, I'd just strip and clean everything - and then clean it again - with special attention to the oil tank, lines, drillings, traps etc and liberal use of compressed air, have the rods checked for ovality at the little ends, fit new bushes and ream them if necessary to get a no-play sliding fit (which takes just modest finger power to push the pin in and out). Plus check the gudgeon pin(s) very carefully for damage on any bearing surface, ditto piston. Replace as necessary, erring on the side of paranoia.
Reassemble and cross fingers for better luck. The which I wish you!