In the Witcher, the only thing we learn about the Conjunction of the Spheres is from the eponymous book found in both Witcher games:
"A cataclysm which occurred 1,500 years ago, trapping in our dimension many unnatural creatures, including ghouls, graveirs and vampires. These beasts have no ecological niches of their own and are merely relics of bygone times.
According to elven lore, humans arrived during the Conjunction, their own world having been destroyed. These human ancestors learned how to harness the power of primordial Chaos, and thus the first human wizards were born. Looking for a place in the world, humans took up arms against the Elder Races, who were unable to withstand the barbarians and ultimately surrendered. This is how humans came to rule the world."
a slightly different version can be found here:
"The cataclysm commonly known as the Conjunction of the Spheres happened one and a half millennia ago. A cosmic collision of several parallel universes ... ... The elves claim that humans also arrived in this world during the Conjunction. This occurred soon after they managed to destroy their own world."
- What caused the cataclysm? It seems that the humans destroying their world might have something to do with it.
- What is the relation of the Conjunction of the Spheres with primordial chaos, which "became more prominent in the world" post-Conjunction?
While this hasn't been revealed in-game, I was hoping that the novels may offer us some clues.