Eric, the most common cause is reloading a cartridge without putting in any powder, just the primer and bullet.
Even the most systematic reloader can make a mistake like this, especially when distracted.
It's one reason I never shoot someone else's reloads, and don't buy commercially reloaded ammunition.
I have never reloaded a squib round like this, but I'm ever vigilant to prevent one. This included always double checking every single reloaded round on my progressive press.
I even installed illumination to make it easier to see powder level:
https://www.youtube.com/watch?v=hIraXq6u0VY
__________________

Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.