“It was humbling, because I thought I knew a lot more about making movies than I did.” — Anne Hathaway recently quoted in the New York Times
It's not even wrong per se; too many writers, editors and publishers have been employing it for too long to call it incorrect, although two of my linguistic professors in college prescriptively decreed that a comma before because, or any of the other subordinating conjunction (as, since, if, et cetera), was verboten. One begrudgingly relented with the so called "contrasting comma," the comma as used earlier in this paragraph before the subordinating conjunction although. Indeed, some grammar books I did find that discussed the topic (including the MLA Handbook) said a comma before because was unnecessary.
If you don’t see a problem with a comma before because, you probably believe it is O.K. to use when what follows because is nonessential to the meaning of the sentence. This is what the Gregg’s Reference Manual also maintains.
The Associated Press Style Guide doesn’t mention its usage, but the Chicago Manual of Style does, calling for a comma when it can drastically change the meaning of the sentence:
She did not love him because he was rich.
She did not love him, because he was rich.
Here, the first sentence implies she did love him but her love was not ensuing of his money. The second sentence says his money was the reason that she could not love him.
As a result of this possible confusion, many prescribe watching out for independent clauses, the first parts of these sentences, that are negative in meaning. The negative or restrictive sense can even come without the use of negative words:
Few adult Romanians speak English because it was forbidden during the Ceausescu era.
I’ll quote Daily Writing Tips, where I got this example sentence: “This sentence, as (not) punctuated, absurdly implies a meaning of ‘This is not the primary reason adult Romanians speak English,’ accompanied by the expectation of a follow-up sentence identifying one or more other causes … Insert a comma, and the sentence tells you what, and then tells you why.”
I think my linguistic professors would call this hogwash as sentences don’t normally reside in isolation and the surrounding paragraph would be enough to clarify its meaning.
Me? I see commas differently. It is neither with the accompaniment of a breath or pause as if the reader and I are on the same cadence nor as a way to break the sentence up into small, bite-sized pieces. This may be why I do not stray too far from my professors’ edicts but sometimes relent and do add a comma to show great contrast, to indicate a supplemental or appended nature, or, as Chicago recommends, to prevent confusion. Of course, I (cursedly) will continue to look at why other writers add commas before subordinating conjunctions, too.