First, Washington has _four_ peaks (not two) over 10,000 feet elevation (including Rainier) and another 10 (if I counted correctly) between 9,000 and 10,000 feet. Timberline in the WA Cascades is about 5,000-5,500 feet, so conditions there can be comparable to 11,000 feet in Colorado.

Second, you don't have to be caught in a blizzard or in below-freezing temperatures to get hypothermia. All you need to do is get sopping wet with no dry clothes or means of getting warm. My daughter got to the incoherent stage at a temp of about 60* while hiking a river-valley trail through waist-high wet vegetation in blue jeans. Her brother and I set up camp in a hurry, got her into her sleeping bag and filled her with hot cocoa. She learned a good lesson about fashion vs. safety. I learned a good lesson about persistence with stubborn teenagers!

Third, I've encountered frost and snow (not a lot, but enough to make things miserable for the unprepared) on high Cascade trails in July and August. While the weather normally isn't as severe as in the Rockies, it can be severe enough to do the job! I would never go out without being prepared for temps in the upper 20's. Overkill 90% of the time, but vitally needed the other 10%.

The higher glaciated peaks such as Mt. Rainier, are well-known for making their own weather, and such things can happen very suddenly. There was a winter storm watch for the Mt. Rainier area (in fact, the whole Washington Cascades) in effect before these folks started up. Whether they ignored it or neglected to check the forecast, we'll probably never know.