The PlayHaven employee was fired because of this event [1]. What led PlayHaven to conduct the investigation? Adria's tweet and blog post to thousands of followers. This isn't the first time she has used her considerable influence as a weapon[2].
It's simple, really. You start at the result - the termination of someone's job - and work backwards. The first action thats absence changes the result is a direct cause of the result.
A direct result of the tweet is the termination of the employee. Without the tweet the parties involved wouldn't be identifiable, and no one would have lost their job.
Is the joke a direct cause of the firing? No. Many subsequent actions must take place to get from the joke to the firing. The joke isn't even required in the sequence of events. Because tweeting a picture of someone and labeling them a sexist is enough to get them fired from a certain employer.
The person being fired is a direct consequence of her tweet and/or blog post. She has thousands of followers. It isn't likely she thought nothing would come of it.
@knowtheory The person being fired is a direct consequence of her tweet. While it isn't likely she directly intended for anyone to lose their jobs, it is equally unlikely she expected absolutely nothing to come of it.
[1]http://blog.playhaven.com/addressing-pycon/
[2]http://amandablumwords.wordpress.com/2013/03/21/3/
Edit: Changed a link.