It was an old school revolver where you have to manually cock back a spring loaded hammer. If you pull it back part way and then gently guide it back to normal position, the firing pin will just rest against the primer (the part of the round that sets off the gunpowder) and nothing will happen. If you pull it all the way back, the hammer locks in place until you pull the trigger, at which point the locking mechanism is unlocked and the hammer is freed to slam the pin into the primer, firing the round. The problem comes if you pull it most of the way back and then lose your grip. In that case, the hammer slams into the round just like if you fired it. Because of the physics involved with pulling back the hammer against a heavy spring (ironically a safety against kids pulling it back), the end of the gun usually gets levered upwards during the act of cocking. So, even if you started pointed directly at the ground, you often won't be by the time the hammer locks in place. It's your job as a gun owner to make sure that nothing you don't mind shooting is in front of the gun at any point during that arc.

Add to this that it was a blank round, meaning there was just gunpowder but no bullet. Usually in a round, the gunpowder is trapped between a big slug of lead (the bullet) and the primer. In a blank, a thin layer of paper and glue is used in place of the bullet to keep the powder from falling out. A lot of people think blanks are 100% safe because there's no bullet, but at very close range that tiny bit of glue still gets shot out with enough force to penetrate skin.

Thus, the guy is still an idiot for pointing the gun in an unsafe direction while cocking it, even if it's a blank, but it's easy to see how a 62yo could lose his grip on the hammer and have the gun go off accidentally in a direction he didn't intend. And because it was a blank, he likely wasn't following full gun discipline like he should have been. This doesn't excuse his behavior (gun owners are literally taught to treat every weapon as loaded and deadly), but it might explain both his behavior and why the article chose the passive "it slipped and it shot" voice. Because basically, he was getting it ready to use as intended and it did magically "go off", and it also is quite possible that it wasn't pointed at the kid when grandpa started the task.