I'd imagine that by bubblehammer he means chunks of the warhammer world existing in isolated pockets wighin the warp, occasionally coming into contact with one another - not so much bubbles crashing into each other as poctets of reality temporarily coming into alignment, imagine unsuspecting empire cities just appearing in the middle of great skaven warrens, or a vast necropolis manifesting amid an elven forest, disgorging a plague of undead that vanish as quickly as they appear, or a tribe of orc survivors praying to gork for a worthy battle, only to suddenly find themselves on a great battlefield of ghost warriors forever fighting the final moments of the end times
Planes of reality coming into and out of alignment, or battling waves of daemons whenever the bartiers protecting them from the warp falter, their mages scrying in the hopes of anticipatjng, preventing, or controlling the next conjunction.
I'm not a fan myself, but whatevs.