True but lead fouling in the barrel and failure of the bullet to grip the barrel rifling (called "stripping the rifling") and subsequent bullet tumbling in flight and complete loss of accuracy, occur at much lower velocities than what it takes to make a bullet tear apart so you rarely get to that point.
In most high speed rifles you have to use some sort of gas checks to shoot cast bullets, don't you? This grips the rifling and keeps the bore sealed. The only rifles where you don't "need" some kind of gas check seem to be in calibers like 45-70 which throw giant slugs at low speed.
I think what happens at the higher velocities to cause tumbling and loss of accuracy before the bullet frags is the bullet melts. When the bullet gets pushed that hard/fast down the barrel, the skin of the cast bullet melts and sheds after exiting the muzzle. Depending on the caliber and twist and alloy, one (fragging vs melting/shedding) might occur before the other. I'd say, usually the melting happens, first.
If you have a muzzle brake on your rifle, you might notice the point where you start losing accuracy and getting tumbling with cast bullets coincides pretty closely with the point where the inside of your muzzle break starts to get coated in lead (in a thin even, shiny silver coating; it makes it look a bit like the black oxide coating of your muzzle brake wore off and underneath it was machined out of aluminum.) If the gas seal/check had failed, the entire bore would be plugged with lead.
I've only read about bullets spontaneously exploding. From the description, it seems like this is secondary to the bullet destabilizing, probably because parts of it melted off. If it were strictly due to centrifugal force overcoming the material, the bullet would explode as soon as it left the muzzle. More often, it seems the bullet goes some amount of yards downrange, first. After it destabilizes (after asymmetrically shedding some melted/softened lead) and starts spinning out of balance is perhaps where they usually explode. Well, I mean, if the bullet explodes in any way, it probably also melted would have melted and tumbled at at even lower velocity.
I have never used a chronograph, but according to reloading manuals, I probably get around 1800 fps with cast and checked rifle bullets. It's fast enough you can't shoot steel targets. They don't get dented; the steel vaporizes on impact.