Well, something is destroying your wheels.
I agree that 225lb is not Automatic Wheel Destruction zone; lots of guys, including myself, ride in that weight zone, on all kinds of wheels, including on lightweight, stock, and low spoke count. Therefore, let's rule out wheel design as a factor.
Similarly, 28c tires are quite fat, and again, experience and observation shows plenty of folks riding the aforementioned wheels with 25c and 23c tires, so let's also rule out tire size.
With regards to pressure, 100-110psi is also common and not particularly high, so not a factor, either.
Considering speed, certainly rotations are stress cycles, and velocity amplifies forces, so that's a likely causal factor, especially since I don't think 40-50mph are common speeds, so it's hard for me to asess the impact. It doesn't seem like it would be a definite factor though, as duration at those speeds would be important to consider, but again, I don't know. 40-50mph every ride may be, literally, the break point, but I've just not heard of guys in the mountains or with big hills destroying wheels at advanced rates, but perhaps they take it for granted and don't complain...
The last big factor is technique, which we know certainly makes a difference. Most of us know guys who do the same rides we do, but who manage to break stuff all the time. Luck is always at play, but I think there are techniques, such as weight shifting, hopping, smooth pedaling, timing, and knowing how to "flow ride" which aren't getting employed in those cases. Just yesterday, I was noticing that one of my clubmates, though he has probably 20lbs on me, is never up front on the descents; why? Is he he trying to slow and driving the wheel into road irregularities? And since he's not OTB, when is he regaining time? Hammering up the hills? That's not flowing; I build momentum when it's easy, and let it carry me down the road or up the next hill until it catches up and feathers into a fairly quick leg speed, so I don't have to jam on the pedals to get back to driving.
Does that kind of stuff reduce spikes of destructive force into the wheels, drivetrain, and other bits? I dunno for sure, but I think so. Something lets me get +4k miles (so far) out of 1372gm, 18f/24r spoke wheelset shod with 23c tires at 220lbs.