I have never had a problem with MagTech ammunition in .44 magnum, .357 magnum, nor 9mm. Yes a squib round is round that does not clear the barrel. It is usually the heartburn of those that reload and often the result of inserting the primer, but not getting the full powder charge in the case before seating the bullet. I had one once when I tried using a Lee progressive press. I stopped using that press.
The squib shot was very, very soft and very quiet. It was so unexpected that I stopped shooting and ran a rod down the barrel, which established that there was an obstruction about 1/2" into the barrel.
Cowboy loads are light loads. One example is in .357 magnum a 158 grain lead bullet standard load might use 6 grains of Unique, while a Cowboy load for the 158 grain lead might only use 4.5 grains.
So while Cowboy Action Shooting loads are lighter than standard loadings, I do not think it that much lighter that one would take great notice of how soft and quiet the discharge was. I am inclined to think squib with one, 'Yeah but'. That being having more than one squib round in a box of factory ammunition. I would think that is an unrealistic assumption under any conditions.
How old was the ammunition ? How had it been stored ? Any chance of damage to the rounds prior to using ? It certainly strikes me as a very odd situation with factory ammunition, but not out of the realm of possibility with hand loads.