Note
I spent most of the day writing at a nearby coffee shop. When I left and walked over to my bike, a guy followed me out and told me that he and his friend watched a man walk down the street and shove the bike (with the baby seat on it) to the ground, for no apparent reason, and then walk away. The two guys went out and picked it back up, but didn't know who the bike belonged to.
All the mechanicals seemed in check, but Z's seat was a little shaky. Taking it off and reinstalling seemed to restabilize it.