One major safety reason rangemasters want slow steady fire is to give you a moment's time between shots to have a chance of realizing a SQUIB round happened and is stuck in the barrel of the weapon - so there's that.
SQUIBS can happen if an oversized bullet is fired and cannot clear the barrel.
It can also happen if a smaller sized round is fired and, as the unsupported case ruptures, the round turns sideways and lodges in the barrel - but this is EXTREMELY rare. But I've picked up severely bulged 9mm cases fired in .40 weapons and the bulge is amazing. - see the attached image.
A SQUIB can happen if the round is extremely undercharged. There are those who say that a primer alone will move a bullet through the barrel but I don't believe them nor will I ever opt to test this theory myself. I never use a powder dropper. I digitally measure each powder load to maintain consistency and for safety. I've never produced a SQUIB round that I know of but know several people who use powder droppers that have on rare occasion. As this will never be worth a finger or an eye to me to risk, I don't use a dropper.
And there is the possibility of squib rounds for those using partial metal jackets or sabot ammo where either the jacket or the outer sabot ring, whatever that may be, get's stuck in the barrel causing the next round to SQUIB. This is exceptionally rare but can and has happened.
And for those doing lead casting - leading can be a problem eventually leading to a squib if you do a LOT of soft lead over time in hot loads and never properly clean your weapons. This is most often a problem for people who read no safety instructions but shot a lot of cast reloads cheaply but rarely harden their alloys. As even these people will fire some FMJ rounds to clear the worst of this leading, this turns out to be extremely rare as well.
Getting back to comments I made in another thread - this sort of discussion is notably missing from the vast majority of reloading manuals and printed product instructions and is typically only discussed in forums such as this or the odd very rare reloading class if you can find one.