In part, it happens because any time one message is being amplified, others get lowered. Someone's message is less likely to be trending if someone else's is. There's a limited number of spots.
Some platforms also have what's called “downvoting”, in which users not only can boost a signal of one thing but can also say that another thing is less relevant. If you have a savvy group that is boosting the signal of the harassment and, when the victim and the victim's allies are trying to get a message out, they are downvoting that, they can essentially be working in both directions. We did see that to a certain extent with the “Gamergate” situation, in which women were being harassed in the games industry. Some of those techniques were being used, although they weren't quite as technically sophisticated as what we've seen recently with the fake news situation.