Got an update this morning on the rider that crashed in front of me yesterday, and she's hurt pretty bad:
"Babs, the bicyclist in the rear, went down and as I understand had multiple fractures of the pelvis, two broken ribs, a punctured lung, and a concussion."

She went down when her front tire hit the back tire of the rider in front of her in a pace line. Babs and the other rider are both experience riders too.