Some rock gets pushed outward sideways. The beveling likely occurred before the stones were forced outward from the lava, cooling against earth and other stone, instead of cooling faster in the open air.
I would be interesting to see what’s under the ground that went along and away from the wall. The wall, whether exposed to air or earth, likely formed a semi-solid edge that directed the molten rock behind it to flow along it as well as in the other direction.
That molten rock that was behind the wall would have pushed the blocks out to different lengths depending on the pressure behind each block and the friction with neighboring, still-cooling, blocks.
What do you think of Nachoman’s magnetic alighnment test?