If there is a building in the direction you want to go it will prevent you from going any further. Like, come on kid, you think you can just walk right through a building? Don't make me laugh!
The building is preventing you from going the right direction, so you pull a bowling (ぼう) ball out of your bag, aim, and bowl the building over. The building, and all the ones around it, go tumbling down. But now the rubble is preventing you from going further.