Durango Freeze Plugs Rusted

I have a 2000 Dodge Durango with a 5.2L Magnum engine. It has 42,000 miles. On March 29, 2005, it started leaking anti-freeze coolant from
the bottom of the truck. I took it to the dealer (since it might be a warranty fix), and they indicated that 3 of the 16 freeze (core) plugs were leaking due to rust. Luckily, this is still covered since I am under the 50,000 mile warranty for this part. My problem is that I am having a hard time understanding how could this rust so easily and so quickly. I have never put water in the radiator well, just coolant a few times to top it off. I have never flushed the system since it's not time yet and I follow maintenance plan which says to do this at 50,000 miles. Is this a design problem with the SUV Truck? Thanks for your responses in advanced.
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