I'm editing the OP because I've since had the mechanics explained to me, so I think it best for this post to exist as a demonstration:
Situation 1: Lemming A builds, Lemming B builds
immediately behind lemming A's bridge:
RESULT: Lemming B does not turn, and simply builds a bridge immediately behind Lemming A's bridge.
Situation 2: Lemmings A builds, Lemming B builds
directly on top of Lemming A's bridge, creating a 2px "wall" at the start of the build structure. Lemming C builds into that structure:
RESULT: Lemming C builds 1 brick, then turns.
Situation 3: At the correct release rate, Lemming A builds, Lemming B
walks over Lemming A's bridge and immediately begins building as soon as he steps off the bridge:
RESULT: Lemming B builds 1 brick, then his terrain check is activated by Lemming A's bridge (still being built behind and above him), so he turns.