The kaiju inside Kafka was specifically looking for him. We don't know since when, but given that it found Kafka right after he saved Reno, we can assume it somehow located Kafka after that one incident.
The kaiju inside Kafka was on a mission to kill kaijus, because of the way it reacted to Isao when Kafka lost control. (It clearly says I'll kill monster which means it was a desire of its own, not just a Kafka projection).
Kafka saw a ghost from the past during his visit to the shrine for kaiju slayers.
Which is odd in itself, but later we are told that numbered weapons users can see the ghosts of other users. It's caused because they are using the memory of the identified kaijus as the core of the weapon.
There are living numbered kaiju weapons, like number 10 with Hoshino. Yet, technically, the Kafka situation is a sort of living numbered kaiju weapon too, except we're talking about a deeper symbiosis.
Kafka is slowly turning into a kaiju and he spends too much time on his kaiju form, he risks not being able to turn back to human.
Whatever is no.9, it has hundreds of kaijus on the inside and apparently it has devoured other kaiju slayers along the eras, not only Isao Shinomiya. Which leads off to think 'bout what is going on and how old is that kaiju .
If the kaiju within Kafka is acting similarly to the way the cores of dead kaijus turned numbered weapons acted, then Kafka saw the ghost of the shrine through the memory of the kaiju within him. It means that the kaiju is pretty old, having been around for a battle on the Meireki Era.
Whatever No.9 is made of is also old, if not older. It's been fighting kaiju slayers for a while, eating them even.
Unlike other kaijus and excluding no.10 (it'd fight anything that moves), no.8 is fighting against its race on its own accord. It looked for Kafka, not the other way around. When did no.8 evolve to understand humanity? When it decided to turn against kaijus and so it'd help exterminate them?
There's a possibility no.8 had done it before— fight alongside a human to destroy kaijus. There is a possibility Kafka is not the first one, but it was chosen by no.8 because it reminded it of another previous human.
There are many ways it could go, but what we are sure of is that Kafka becoming no.8 user is not a coincidence AND the Defense Corps don't know half the story yet.
( I personally think no.8 is a part of the actual monster within no.9 that escaped and now its trying to kill it, maybe motivated by the memory of the kaiju slayers no.9 real monster has ate )
