Ok causes for most squibs that I can think of off the top of my head.
1. Somehow missed charging the case. With a progressive if one gets into la la land for a tad bit of time it can happen. As well as bridging powder with some types of stick, and large flake powders.
2. Bad powder. If the others fired then that would not be the case.
3. Clogged flash hole. Some times dirt clods can work their way into range pick up brass. The decapping die pokes a hole into it. Somehow during handling a piece of the dirt makes its way to the flash hole. Or impacted media. Especialy corn cob media in deprimed brass in a tumbler.
4. In rifles, and magnum rounds with slow powders. Not enough case fill can keep the powder from igniting.
6. In magnum hand gun, and rifle rounds using a non magnum primer for slow powders can fail to ignite the powder, yet unseat the bullet causing a squib.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.