Gary is right to advise you to start at the beginning of this thread.
Some will present a technical definition, but in general terms, KB means - KA BOOOOM - Blowing up the gun - Blowing apart the gun - Very bad if you are close to the gun - Injury or death can occur. There are many many reasons for a KB.
There are also many many reasons for over pressure. I would add to the above that over pressure can also be caused by the over all length (OAL) being too long which engages the bullet with the lands and groves of the rifling before the detonation of the round. Loading the wrong (too big) diameter bullet can also cause over pressure. Very fast powder in a big case and too much air or space combined with a heavy bullet might cause over pressure.
Rounds that do not fully load into the chamber and the gun (defective perhaps)still allows the round to be detonated causing an explosion. This may not technically be over pressure and it may not technicly be a KB (depending on who supplies the definiition), but there is an explosion and the explosion is not contained because the case is not supported. The case is ruptured and may cause collateral damage. If you are the one holding the gun at the time, you will hear KA BOOOM and you will be lucky if no one is injured.
If you are reloading, you need to follow the recipe. You need to be very careful and remember you are dealing with a controlled explosion in your hand that is not far from your head. The "Myth Busters" on the Discovery Channel did a show on plugged shotgun barrels with interesting results.
Some people have bad luck and report a KB even when they act cautiously with a modern gun and new factory ammo. KBs have been reported with many brands and types of guns and ammo. Some may be more susceptible than others.
If you are using factory rounds you need to make sure your gun is in good working condition and designed for those rounds (some rounds are marked as high pressure or high velocity). Some guns, depending on age, condition, materials, design and etc, etc should only be fired with lower pressure rounds. Some are not safe to fire at all. Guns are mechanical devices and like all mechanical devices need to be properly inspected and maintained but all will eventually fail.
Some KBs have been reported with "old" factory rounds. Yet many people shoot "old" military surplus regularly without problems. Others report KBs with rounds that were found in an old drawer or box. A cutious person would not eat a pill he found lying around because he does not know what is in it. The same should apply to ammo.
Some people like to test the extremes (like the guy who fired an elephant rifle round in a modified handgun ;g - reportedly lost part of his hand and it my not have been a KB). Others do not use common sense and caution;P. Some in both groups have been seriously injured or killed.
BE CAREFUL - BE SAFE
FREEDOM IS NOT FREE
NRA ENDOWMENT LIFE
GOLDEN EAGLES, ILA