Suppose someone does something a minority dislike and the majority are indifferent towards. Once the minority has struck out at the person, many of majority start striking the person out of a sense of community catharsis or joyous solidarity.
What would you call this sentiment or behaviour?
For instance, I see someone post a bad answer on a (different) stack exchange. I withhold downvoting, but then one or two people down-vote and harangue the poster, at which time I and many others start downvoting in a veritable flood; with a sense of warped pleasure that that poor poster isn't us. Isn't One of Us, at least for today.
Best Answer
I think it is bandwagon effect.
Image source: https://sites.google.com/site/apgovvocabwiki45/unit-3-terms/bandwagon-effect
Bandwagon effect shows itself in negative situations too, of course. It is often mentioned along with your very own example about crescendo-like downvoting ceremony. [See: Google Search] [Your downvoting example is even used as bandwagon downvoting in internet jargon]
Some examples from online communities (first one is from our own meta stackexchange):
As a bonus, some studies about the topic:
How Community Feedback Shapes User Behavior
Data Mining Reveals How The “Down-Vote” Leads To A Vicious Circle Of Negative Feedback