It's not that its "directional". It's that it is sitting directly next to the slab. If it was a non-directional bomb then it would also explode outwards in the other open air directions (which wouldn't leave behind any rubble). The explosion dissipates super fast the farther away you get meaning the only thing that gets blown away is the stuff right next to the explosives.

Secondly, it could be a directional explosive. They could have made it with some sort of shape charge like they do in demolition which are directional. Either way though, the debris pattern would look similar for both in this case, they could have pulled it off with a weaker set of explosives if they made it directional.