A similar discussion came up on a bass fishing site. Most baitcasters & spinning rods these days are graphite, just as fly rods are, and apparently there are some issues with breakage with the first use. IMO, it could simply be a handling problem. No matter how well a manufacturer handles a rod on their end, once it leaves them, it's at the mercy of shippers & others who have to handle the rod until it's at the retailers. Even if the rod is sold direct, their is still a degree of handling, and again, if shipped, at the mercy of the shipper. Shippers are not always kind to anything they handle!
Graphite & the various other hi-tech materials used in fly rods & other rods are relatively brittle & fragile. They can take stresses for which they're designed, but not impacts.
I've been fortunate with rods so far. I have many, including baitcasters, spinning & trolling rods, and have only had issues with 3. Two arrived damaged, clearly the result of poor handling, and one broke (a spinning rod) the first time I used it. I broke an old fiberglass fly rod I had in a car door, my fault entirely, but that's a different situation. I had used that rod for many, many years prior to breaking it. Got in a hurry, got careless. My fault!
Also, on another site I frequent, where the majority of the discussion is about fishing the Chesapeake Bay, there have been discussions about rod breakage. I drive a truck for a living & am on the road locally a lot, so I often see boats being towed with rods in holders & big lures, hooks & hardware flopping around against the sides of rods. IMO, this cannot be good for a rod, and when someone says, "hey, my rod broke, only used it a few times!" I can understand why such a problem exists. The old fiberglass rods took a lot more abuse!
IMO, short of an actual defect, which can occur, my guess is that most breakage is due to either poor handling, or incidental damage.
At least that's my theory!