Sometimes a mundane explanation doesn't immediately come to you.
It occurred to me that the source of the new seedlings could be partly my own doing.
It occurred to me that the source could have been me or others picking up seeds in the soles of our shoes, which scarified them enough to make them sprout after they rubbed off our shoes and went into the soil.
I asked whether that was possible and was told it was. I guess more investigation could tell us more.