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 occurence with thin and long case case necks such as the 30-30 Winchester. Be sure to set the die per instructions. For a clever way to set this die quickly, see our FAQ under "Dies and Die Questions" and then "Seating Die Adjustment".
The second reason is generally associated with our 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.