Only a wild thought...don’t suppose the grease nipple was replaced with one with too long a thread, which then rubbed tight against the king pin? (This issue is referred to in gomog.) If it happens again, may be worth unscrewing the nipple to see if anything gives.
Yet another thought...the fact that it released when unbalanced force was exerted on the cross frame as it was lowered with one wheel on the ground, could it be due to a broken or maladjusted diagonal brace rod. (the rod that connects between the top of the cross frame with the lower end to the chassis. Not unknown for these to fracture.
