I believe age and accessibility has something to do with it.
A lot of teens (And some adult unfortunately) are rather selfish and kind of arrogant. They have yet to understand how important courtesy is. Especially with those special flakes who think they are so special and unique.
Combined this with how easy it is to access internet today, and voila, you have waves of insolent brats online.
Bad seeds always exist, but the rise of internet has made their presence that much more noticeable nowadays.
I agree with what have been said so far, and I would like to add the distancing that happens with online vs "Face-to-face". That are some nice studies about it nowadays.
In a way, people would not say bad things face-to-face when talking with someone else because they feel accountable for what they are saying, which does not happens online.
Of course, that's not the only answer for those acts, but I find it very interesting to point out.
Being a courteous person shows that you undertstand a person from their perspective. I am very respectful when someone tells me something. I appreciate when people help me. It is very courteous of them to help me.