Sounds like you might have got a leaner rack this time. A Costco rack of ribs (I call them the "hormone special") is enormous, with lots of meat. Ribs from, say, Whole Foods are smaller.
Either way, fat content varies so you might have gotten a leaner rib that may have dried out.
The collagen in the ribs will start to break down at about 160 degrees. The heat energy going into the ribs will continue to break down the collagen as opposed to raising the temperature of the meat. Once the collagen has broken down the meat temp will start to rise again.
As for where you went wrong, not sure, but there here are some ideas. 250 degrees is a bit high for a long dry heat. I'd go with 200, keep it below the boiling point. If you go higher, put a pan of water in the over to keep things moist. You may have just dried out the ribs.
Restaurants and home aficionados will often do ribs in three stages: smoke, steam and then grill. The smoking provides the initial flavor, the moist heat from steam quickly breaks down the collagen and then grilling caramelizes any sauce and makes the wonderful crust. The steaming part is easily done just by wrapping the ribs in foil and putting them back on the smoker or the grill.
Any way you cut it the trick is to break down that collagen before the meat dries out. Look for a final meat temperature of around 195.
-Stooxie
