They don't seem to follow a pattern.
First runthrough, the bottom wall housing the group of enemies fell when I got past it and flanked my back-liners.
Second runthrough, I put all my strong units in front of it... and waited... and waited... and nothing. Then I moved on and the walls came down to let the dark knights flank my fragile units. (I guess it's my fault for not counting squares).
Third runthrough, I went as fast as I could, wall fell soon after, letting them kill my fragile units.
Fourth runthough, I got near the end, and was wondering why the walls didn't fall. Suddenly they fell all at once. (I think because I unlocked the door after waiting a couple of turns with nothing happening)
Last time, I just gave up with bringing untrained units to level up and raped the chapter up the ass with my maxed out children of doom.