Except for blockers, lemmings don't interact with each other, e.g. they interpenetrate rather than collide. The lemming world has kinematics but apparently not dynamics. Thus the program uses velocities but not forces and accelerations. Our own projections of plans is rarely even kinematic but merely geometric. I mean that the velocities of the lemmings are not used in projection.
Lemmings fall at constant velocity but are destroyed if they fall from too great a height.
Bridges are infinitely strong and don't break under the weight of any number of lemmings. Explosions kill only the lemming that has been told to explode. Others are unharmed even if they are within the explosion zone. There seems to be no random element in lemming games except that randomness coming from the human player. As is usual in sports, this permits the human to learn precise procedures from repeated plays of a game.