Re-posting from the social. I'm talking about different times that the bomb is active and inactive.
Once the bomb is stationary (inactive) it will no longer detect hurtboxes, even if you make it move again by pushing it off a platform with your character model or pushing it with a windbox etc. The bomb can also become inactive momentarily if it is hit by something not powerful enough to make it explode, it's like the bomb can go through hitlag and hitsun. As for bombs hitting shields... that's a bit more complicated. In fact it's really complicated, for no apparent reason. This is one time that I'm happy people don't bother reading through my long posts XD, because what comes next isn't very good news for us. When a bomb hits a shield, the bomb doesn't become inactive perse, or even necessarilly as I'll explain further on, but in general, yeah, it becomes completely inactive for the duration. The bomb will always be able to hit other opponent's hurtboxes once it bounces of someone else's shield, but it actually becomes 'inactive' for the person who shielded it, with one exception which I'll go into in a second. This means that in most situations, if your opponent shields the bomb, they can have it inside their character model after it has bounced upwards off their shield, as it is coming back down, and all the time while it is bouncing on the ground. So don't throw the bomb at people's shields unless the fuse has been cooking for a bit if they are aware of this, as it basically means, hey, here's a bomb, you can either ignore it or feel free to throw it back at me. But then, what happened in the vid, right? The bomb was shielded, it bounced on the ground, and then it blew up when it came into contact with Sheik's hurtbox before the timed fuse was up, so how do I explain that? Well here's where it gets silly. If the opponent power-shields the bomb (not the bomb blast, but the bomb itself), the bomb will remain active against them. In other words, people get punished for power shielding because Sakurai said so. If you want to test it yourself, try it against an opponent who shields out of crouch and throw it down at them as this makes it easier to power-shield the bomb. This is one reason why Gay's bombs can actually be better because if his bombs are shielded with a normal shield, they will at least explode when they hit the ground. Link can make up for this though with his shorter fuses, so a lot of the time Link's bombs will work out better even if they are shielded normally because they will often explode before they even touch the ground.