Maybe to much water or a bad hard freeze. The hardest thing to do is not water your sedums. I had 2 real hard freezes that lasted for days and I lost my cacti but not the sedums.

Rough Spring for everybody's gardens this year, I think. Freezing temps like robin303 mentioned, then up in the 50's, then down again into the teens. I'm going to have to replace what was well-established plants,too. But, that's gardening.

I know, it seems like the so called "mild winters" can sometimes be worse than the "hard winters" due to the freezing and thawing. Usually if they stay frozen all winter and have a snow cover they are fine. Well drained soil is important as many of the plants your talking about don't like wet feet in winter. I lost a lot this year too. I would move anything "portable inside the garage for winter or I see that the folks at the flower factory in Madison wrap their large troughs in plastic, more to keep them from being too wet than to keep cold out I think.
Catrina