I let this hybrid Jules x huanucoensis (or reverse) get so root bound that it sent out roots into the air in hopes of finding water in the walls of my my house.  As soon as I got to the new place, I repotted most of the cacti.  Several of them sprouted base pups.  This plants so young, I am surprised.  I think the season or two of water starving it, followed by heavy ferts and a bigger pot... Triggered pups.  All three cacti I had with roots growing out the side have pupped since repotting.