Upon ignition, the case expands in all directions, with the case walls tightly gripping the chamber. Almost instantly, the case relaxes its grip on the chamber walls as the pressure starts to drop, allowing the case to be extracted. If the cartridge were lubricated, the case wouldn’t be able to grip the chamber walls, allowing it to slam back sharply against the bolt face. In extreme examples, this force, called “bolt thrust,” can even damage a firearm. For what it’s worth, the British used to use lubricated cartridges to proof test a firearm, rather than the so called “blue pill” loads used here in the US. Wipe those cases, and make sure your chamber stays clean and dry!