Slow moving forum. Oh well.
I suppose it's also possible the developers were going for a sort of frightening loss of control feeling because you don't really know how it happened. If so, though, I'm not sure something which
actively feels like it makes no sense was the right way to go.
Likely the most accurate answer, however, is just gameplay reasons. It would have been even more annoying if you had to kill an infected that was just standing there as it would have been blatantly obvious what it was. Perhaps they could have made you walk over to the lever that drops the crate and then turn around to see an infected running, and Clarence says "Oh no, we're trapped! Quick, do something!" That would be glorious. You'd get a moment of "Haha, the switch! I'm genius!" followed by "Oh. Oh my. Whoops."
Minor imperfections on an otherwise excellent game, though seemingly a little rushed at the end. Knowing that the devs will have learned quite a bit since these games, I'll be even more interested in their later horror ones. Perhaps in the distant, distant future, when technology has leaped forward even more, perhaps they could do a remake of Penumbra.