I think it is bandwagon effect.
The bandwagon effect is a phenomenon whereby the rate of uptake of beliefs, ideas, fads and trends increases the more that they have already been adopted by others.
In other words, the bandwagon effect is characterized by the probability of individual adoption increasing with respect to the proportion who have already done so. As more people come to believe in something, others also "hop on the bandwagon" regardless of the underlying evidence.
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):
Bandwagon effect: When one user sees a post that has a few upvotes or downvotes and decides to cast his/her vote the same way as the popular opinion.
For the first few hours after a submission is created, the score is not displayed. This is intended to mitigate the bandwagon effect.
...the bandwagon effect kicks in and people will have seen the negative score before they even read the comment and the pre-judgement begins to show.
The reason I feel having it visible is wrong is because of the whole bandwagon effect it has on people. If someone sees a downvoted post, they are more likely to disagree or even ignore it because "everyone else doesn't like it so it must be wrong". The inverse is also true for visible upvotes; "everyone else likes this so it must be right".
As a bonus, some studies about the topic: