«شَرٌّ أَهَرَّ ذَا نَابٍ»
Some evil caused a fanged creature to growl.
This proverbial construction illustrates why an initial indefinite does not invariably mean “this category alone and no other.” The primary effect can be to introduce a grave cause or draw attention to the existence of an explanation.
If the speaker's aim is simply to mention a cause of the event, it would be unwarranted to deny every other imaginable cause. Ordinary assertion, restriction, and reinforcement remain distinct possible functions.
Ask: Which actual alternative is the speaker excluding? Without a grounded contrast, strict exclusivity should not be forced onto the sentence.