[noirscape]

It's pretty easy to explain; Apple devices turned the double dash (--) into an em dash by default. (Not sure if they still do.)

You probably won't encounter it in professional(-ish) writing very often (since that's premeditated and usually written on a computer, and most computer users are still on Windows), but in more informal situations (microblogs, text chat) most people don't really care as much to correct the behavior and nobody turns it off, meaning that the frequency of em dashes is unintentionally way higher than it probably should be.