What is a jag sib?

[MPenn]

If that is the case then why do so many jags seem to have this "tick" while relatively few normals have it? What are so many jag owners doing wrong to induce the condition that normal owners are not doing? And why, when jags cost $1-5k a few years ago, were serious keepers doing anything with such expensive animals that might put their animals at risk?

No one is for sure why the jags have this "tick". It is believed to be similar to the spider ball. Both have a lethal super and the het (ie. jag and spider) is a carrier.
I will add that not all jags exhibit this behavior. It has been concluded that it can be brought on by stress (ie. chemicals, high heat, combatting).
I am not sure where you think that jag owners are doing something intentionally to produce bobble-head jags. Read above.