Jen, the pictures look to me like severe damage from fungus gnat larvae. They look like termites and munch tunnels into the root crown and stem right at soil level.
The adults kind of look like little mosquitoes or gnats (they are gnats). They lay their eggs in moist soil. I have. Lost plants to them and it sounds very similar to what happened to you.
One thing you can do is leave the soil at the base of plants unmulched and try to keep it dry. You could also try to keep DE dusted around the base of plants. Another effective option is to water with a BT (bacillus thuriniensis (sp?)) product. That bacteria predates fungus gnat eggs. And lastly, you can get sticky traps for the adults that you just stake up in the garden.
One last thing, and it may be too late this time, you should cover the area around a plant that dies and gets pulled from fungus gnats with cardboard to inhibit the new adults from getting out of the soil and laying more eggs. They're a pain but you can totally eliminate them without gnarly sprays, the garden that taught me all about them produced without damage for several years after the year the gnats wrought their havoc