When you and at least four of your friends carve a pit into the ground and defecate into it, then proceed to mud wrestle in the resulting pile of excrement
“Hey man, me and my friends where gonna go do a San Diego dog pile, wanna join?”
A disagreement on an Internet message board wherein one person says something wrong or offensive, and a large number of people comment in response to tell the person how wrong and/or horrible they are, and continue to disparage the original commenter beyond any reasonable time limit.
People commenting at the same time, without realizing others are jumping on the person as well, do not make a dogpile. The requirement is to join in with an angry group to yell at an easy target, or to get popularity points for being seen to agree with the group. They see that everyone else is doing something, and they copy it.
The original commenter typically does not respond at all, because they are completely overwhelmed or scared off. Once a dogpile has been established, an apology from the original commenter is less likely to be effective.
The same thing as a dogpile, the difference being that it occurs online and with a bunch of SJW's
Doug: Did you hear what happened to Timmy?
Joe: No, what happened?
Doug: He killed himself after an SJW Dogpile got on him for being a straight, white male.