My guess is that aggiegrads and Andrew are correct: fatigue failure.
Stuff responds to stress (force per area, for example in 1000s of pounds per square inch) in several ways.
1) acute elastic deformation. Below a certain stress level the part bends when stress is applied but returns to original form when stress is removed. All bicycle parts should be designed with strong enough materials and enough metal so that normal stress is in this elastic range.
2) acute plastic deformation. Above a certain "yield" stress, the part bends and does not return to original form. Ideally, bike parts won't be subject to this stress but good design could use stress in this region to absorb shock. Car manufacturers design cars to crumple (plastically deform) in a predictable way to absorb energy in crashes. Not sure if bike mfrs design bikes to take advantage of plastic deformation in crashes.
3) acute tensile fracture. At an "ultimate" stress above yield stress, the material has stretched all it can, and it breaks completely. There is no way a properly designed bike part should see ultimate stress in normal use. If you are careening down the Col du Manse at 50 mph and hit a wall or pothole, you might break your form blades off. That would be tensile fracture.
Given that the forces on bicycle parts are pretty much known, and stress is force per area, bike design is simple, right? We just design the part with enough "meat" (metal or CF area) to lower the stress to below yield stress. Not that simple. There is a third threshold which is called fatigue point, or fatigue stress. It is less then yield stress. A part will fail after many cycles of stress that is greater than the fatigue point,. This is fatigue failure. Fatigue in common metals is very well characterized. The higher the stress above the fatigue point, the fewer cycles required to fail. So if a mfr pares down the amt of material in (for example) a rim, the stress the metal in the rim sees is higher and it fails in fewer cycles. Or, if a rim is over-tensioned, the stress level is higher and you see fatigue
So, after 5000km, your wheels have been subjected to about 2.2 million revolutions. Each revolution is a stress cycle. It's pretty clear from your pics and discussion that your failure mode was fatigue. If you had inspected your rims a week ago, I'd bet you would have seen small fatigue cracks near many of the nipples. It may be that these rims were under-built (not enough metal or poor design geometry, with normal cycling forces cause higher than desirable stress). Or the spokes may have been over-tensioned. Higher force, hence higher stress, on the rims. Alternately, if you are a Clydesdale, the forces and stress the bike sees are higher than design (not poking fun of anyone: I weigh 245# myself). If you used the bike to commute every day over very rough roads, that increases the stress and could facilitate failure.
BTW, designing for fatigue strength is the main reason why aluminum bikes aren't much lighter. Aluminum has a much lower fatigue point than steel and you need to add more meat to get adequate fatigue strength.
If 5000km life is good for you, the same rims may be ok. You would want the wheel builder to be scrupulous about spoke tension, and you'd need to be very diligent about inspecting the rims frequently. I myself would try to find something beefier.
BTW, there's another consideration in bike design. It's the reason that the fanciest steel bikes aren't much lighter: Stiffness. All steels are about the same stiffness and this is a limiting factor. For example, Reynolds 953 is a "super steel". It has tensile strength about 2-3 times that of normal steels, and has high yield strength and fatigue point as well. But its about the same stiffness as low-carbon mild steel. So you have minimum tube thickness set by stiffness, not strength. Going thinner and you get a bicycle frame as stiff as cooked spaghetti. And aluminum and Ti are both much less stiff than steel. As stated above, aluminum bikes have thicker-walled frames mostly due to the fatigue limit. This is why Al frames are generally stiffer than steel frames. They have to be thick enough for fatigue strength, and this thickness results in stiffness.