Ignition of powder is based on the assumption the primer is igniting a column of powder that burns from the primer end forward. The small spaces between the grains then limit how fast the flame spreads forward and, therefore how fast it ignites the powder. When a rifle case's fill density gets below about 60%, the powder can lay sloped along the bottom side of the case, exposing a much larger surface area to the flash hole than just the end of a column of powder. That can allow a lot more of it to light up at once, producing the equivalent behavior of a faster burning powder than it would be when lit mainly from one end of the case only. In .30-06, Dr. Lloyd Brownell observed the effect could, on occasion, produce pressures double that of a normal full load.
If instead of slow-burning rifle powder, you are working with small quantities of a fast powder, fast burn rate has already been taken into account, so the ignition position isn't nearly as much of a pressure variation factor.
The above is all an issue with rifle powder and cases. I don't think I've heard of it happening with handgun cases. However, some slow handgun powders, like 296/H110, burn too slowly to remain lit when a bullet jumps forward through the barrel-to-cylinder gap in a revolver. This means that when you underload them, they are not making much pressure when the bullet jumps that gap, and the pressure can occasionally bleed off faster than the powder can make it. This results in the powder burn being interrupted and the load extinguishing and "squibbing out," leaving the bullet stuck in the barrel. The next round fires and runs into that obstruction before the gap is exposed, so all the pressure builds up without the bullet moving forward and making more space. That bursts the gun.
Finally, with all cartridges, if you get powder quantity low enough and slow enough, rather than make enough pressure to keep the case expanded against the chamber, the bullet can get out, and then the pressure escapes backward through the gap in the chamber between its walls and the unexpanded case. This also leaves a bullet stuck in the barrel, which is very dangerous if the next load is normal.