I've been through Ferihegy airport many, many times and don't recall ever seeing any soups, much less mushroom soup, on sale at the duty free shop. On the other hand, I personally know several Hungarians who have smuggled all sorts of banned things onboard (and through U.S. customs), including a three-foot, very garlicy and smoky sausage and about a kilo of fresh Hungarian yellow wax peppers (for making lecso). And so nothing would surprise me as far as carry-on goes.
Perhaps this was homemade mushroom soup, and thus in some sort of plastic or other container and not in a can, and perhaps it didn't just leak out of the bin, but spilled onto the passenger when someone opened the bin and removed/opened the bag containing it, perhaps after it had tipped over during flight/takeoff.
Still, it is an incredible coincidence. But then again, these things do happen.