Hit the nail on the head. People tend to assume something to be their gender when that gender is unknown. You see a doggo? If you don't know it's gender, you may refer to it as a "him" if you're male or as a "her" if you're a female. This applies to anonymous accounts. You don't know who that other person is in any way. So your mind tries to make a connection. And people tend to self-reflect themselves onto others. This is exacerbated by the fact that a huge majority of reddit is young American men, so more often than not, they assume male due to the first point and then that's reinforced by the sheer number of men on the site, so it's often reinforced. You can switch this gender norm around on Tumblr and it would still hold true. Most people there would assume me to be female at first instance.