lol

Chemical bombs don't explode immediately like TNT or Plastique. They have a trigger, usually a small explosive device, and then an acceleration process that can take (depending on construction) several seconds. What is shown in these videos is the result of expanding gases from the initial triggering device. Probably the skylight or air conditioner on the roof, being blown off the roof by the initial triggering device. You can CLEARLY see the projectile going UP.

Then, the reason the smoke trail appears to come back down is because the smoke trail less dense higher up and away from the point of origin. Thus, it dissipates from visibility higher up first as the level of dissipation works back to the point of origin.

But i don't know, maybe it's more entertaining to let our imaginations run wild than consider actual science.