Sentences starting with "Which": Fragment or complete sentence?

I read an article in a newspaper and was wondering if what I read was a sentence fragment.

The sentence/fragment in question is:

Which is why we believe the proposed amendments should be passed.

Wouldn't this be a subordinate clause fragment with "which" being the relative pronoun as the subject and is as the main verb?

Isn't any sentence starting with "Which" a fragment?

Solution 1:

Which can begin a complete sentence, both as a pronoun, and as an adjective.

Examples:
Which sentence is correct?

Which of the sentences is correct cannot be determined.

That said, what you've quoted is not a complete sentence. This style of punctuation - using a period instead of a semicolon - is relatively common in non-formal texts. It makes the text appear more conversational, since we very often add subordinate clauses like this as an afterthought. In written texts, the period either suggests that the clause is an afterthought, or else it adds emphasis to the clause by separating it from the previous clause. Separating it more than a semicolon would.