Why is this that-clause unnatural as the subject in this sentence?

Question

(1)After the surprising Algiers Agreement between Iran and Iraq is reached, the United States stops its support for the Kurdish rebels, which causes the fragmentation of the opposition and an increased vulnerability to Hussein's renewed attacks.

(2)That the United States stops its support for the Kurdish rebels causes the fragmentation of the opposition and an increased vulnerability to Hussein's renewed attacks.

(3)It causes the fragmentation of the opposition and an increased vulnerability to Hussein's renewed attacks that the United States stops its support for the Kurdish rebels

Why are both (2) and (3) unnatural, while (1) is natural?

In PEU Swan writes which can refer not only to a noun, but also to the whole of a previous clause, in this case, the United States stops its support for the Kurdish rebels.

He also writes, in another place, a that-clause can be the subject.

So, there should be no problem with (2) or (3).

Answer 1

There is nothing wrong with using a that-clause as the subject of this sentence. The problem is with your use of the simple present tense, which is certainly possible but seems unusual here due to how we would usually expect these events to be reported. (That also makes sentence 1 seem a bit unnatural to me.) Consider this version of your second sentence:

That the United States is stopping its support for the Kurdish rebels has caused the fragmentation of the opposition and an increased vulnerability to Hussein's renewed attacks.

Voila! Now it's fine.

Answer 2

While some verbs can take a that-clause as a (usually extraposed) subject, most verbs cannot. The verb cause does not allow such a subject.

Edit: Auracaria provides this counterexample:

That you could even suggest such a thing causes the hairs on the back of my neck to bristle with rage.

I'd delete this answer but the comment thread is worth preserving.