I suspect, and this is just from observation and no scientific facts but that cacti seedlings that grow "naturally" germinate when the conditions are just right, and a lot of them like water. I have two Opuntia volunteers coming up where a vehicle used to be parked. (it was a SUV that had high ground clearance and it looks like they sprouted last winter. (not this winter, but last winter, or maybe during the summer monsoons.
Here's to you, all you insidious creatures of green..er I mean cacti.