Dogpiling (Internet)

Dogpiling or a dog-pile is a form of online harassment where a large number of accounts suddenly fill the comment thread of a controversial post with criticisms and/or insults, or target a single person. Some definitions require coordination. In some definitions, it also includes sending private messages. 
History.
The expression has been used in reference to the Gamergate controversy.
Basic information.
Dogpiling is a form of cyberbullying. It can get way out of hand with the large amount of harassment that happens in a big quantity on a single post. It is mainly with negative connotation but also in some cases it can be positive too.