It could be anything from the facility they go through, trucks/roads that they go on to get there, whether they end up being flighted, the weather and many things including, yes, postal workers throwing boxes around.
I will admit that if something does not say fragile, we will do what we have to in order to fit the boxes in our vehicle. I don't mean crush them, run them over, step on them or do anything BAD to them, but we do creatively stack them to get them all to fit in our vehicle, sometimes they might get dropped inadvertently, they might fall off the assembly line in the sorting facility. MANY things could happen. Most carriers will take extra care with packages marked fragile which is why I, as a carrier, always beg for people to mark their packages appropriately, but I know a lot of people refuse to because they think it is like putting a bulls eye on the package.
I totally understand the frustration, as someone that has had a few issues in the past with shipping eggs. Unfortunately, it's a gamble any time we send eggs in the mail (or any way that we ship eggs). I would be curious to know if you have better success shipping UPS. The fact is, eggs really are fragile and were not made to be shipped so it always amazes me when someone gets a full batch with no harm at all to them. It does happen a lot and I have gotten plenty perfect, but it still amazes me because of all of the factors that play in to getting them here.