Final answer:
A squib load in a fired cartridge is most commonly caused by a faulty primer or insufficient propellant, which could lead to the bullet becoming lodged in the barrel.
Step-by-step explanation:
The cause of a squib load in a fired cartridge is typically due to insufficient propellant charge, which can be the result of several issues, such as a faulty primer, using incorrect or too little powder, or issues with the powder itself. It's not commonly caused by an overloaded cartridge, improper bullet seating, or barrel obstruction; these issues generally lead to different types of firearm malfunctions. A squib load is dangerous because the bullet may not have enough force to exit the barrel, leading to an obstruction that could result in a catastrophic failure if another round is fired.