While there can be many reasons for a case to be crushed in the reloading process, there are primarily two.
The first is that the bullet seating die is not set properly. If the crimp shoulder is set too deeply, the case is crimped before the bottom of the press stroke is achieved. Once the bullet is seated, additional pressure pushes the already heavily crimped case into the die further resulting in a crushed case. This is a common occurrence with thin and long case necks such as the 30-30 Winchester or long pistol rounds such as the .357 Magnum. Be sure to set the die per instructions.
This is a clever way to set the Bullet Seating Die quickly. The trick is to set the bullet seating depth first, then the crimp. This is done as follows; With an empty, sized case in the shell holder, hold the ram at the top of its stroke. Turn the bullet seating die body down over the case until you feel it come to a stop. This will be when the case mouth contacts the crimp shoulder inside the die. Mark this position by turning the lock ring down against the turret or press frame. Now adjust your bullet seating depth. Once you have the bullet seated to the desired depth, back the bullet seater adjuster out about 1 turn. Now turn the bullet seating die body in to apply the desired crimp. Once this is established, hold the ram at the top of its stroke and spin the bullet seater adjuster down until it stops. It is a good idea to carry this out with an empty case, so that after you have seated the bullet, you have a perfectly safe dummy cartridge (or gauge) to repetitively set seating depth and crimp on future occasions. Once you have this dummy gauge, all you have to do is place it in the shell holder, raise the ram to the top of its stroke, turn the bullet seating body down until it stops, and then turn the bullet seating depth adjuster in until it stops. Since the crimp and the seating depth were already set, the die will return to very close to the same settings.
The second reason is generally associated with the Lee Collet Dies. If the collet in the die has been collapsed, the case neck can not clearly enter the collet area. This results in a crushed and/or buckled case. This can be avoided by not operating the die without a case in it. If this happens, it is necessary to remove the collet and pry the collet leaves back so that they stay apart when inserted back into the die. The ID of the die body will cause the leaves to stay uniformity apart.