Sorry to rub salt in a wound, but moving a loose load (and unwrapped interlocked boxes that tall are still "loose" since they can (and did) collapse*) that is stacked above handle hight of a pallet truck is asking for trouble.
As my father sagaciously always said: Don't break your back or your heart. If you overload, something will break. Take two trips.
* This is even mentioned by OSHA (my highlight): "Bags, containers, bundles, etc. are stored in tiers that are stacked, blocked, interlocked and limited in height so that they are stable and secure to prevent sliding or collapse."
Man, that stinks. Were those all full? Hopefully most survived.
balatonwine's father has some solid advice. I've never loaded a pallet jack much higher than the handle and that was always with sturdy construction material. If you need to stack that high for whatever unavoidable reason (limited space, etc.), then I would suggest wrapping with the stuff that looks like saran wrap that they use for shipping and putting some sort of support (heavy cardboard) every few rows.
All full. It could have been worse. About 30 cases toppled off when the pallet jack broke through the floor drain. I only lost 33 bottles. I was only moving them a few feet. Live and learn. I should have had the stack a little shorter and wrapped in plastic wrap.
My dad (he's 90 now) tells the story about a very proper lady he used to work. Something horrible happened at work one day and he says all she could say was " Cuss words, Cuss words, naughty words in abundance!!" Somehow, I think much worse than that would have come out of my mouth (and maybe hers) if something like this happened.