He blames grass clippings, but it's apparent after reading the report that the reality is she rear-ended him when he, the leader of a group ride, slowed down while signalling his group to ride single file. When you signal your group to ride single file, changing from staggered formation effectively halves every rider's following distance until they each fall back to a proper distance. By slowing at the same time, he exacerbated the problem, making himself a contributor to, if not the main cause of, her crash.
Totally agree with your assessment. Nothing in the story suggests that the grass itself was an issue.
Grass in the road is a hazard, sure, but so is road kill, chip seal, drivers on the phone, the list is near endless.
Keep your eyes peeled and head on a swivel.