I lost four plants under what appears to be similar circumstances a few weeks back. Not sure what happened... There was some apparent damage the the stems of 3 out of 4 plants, under the soil line and just above the roots. Even more telling, to me, was the fact that all of the plants had what is consider underdeveloped root systems.
Ever since I pulled those plants, I've been living in a state of mild paranoia, wiring that note of my plants will be similarly affected.