[throwaway0321]

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.