What is a word to describe the behaviour where you do something because everyone is also doing this, to the point where you do it without any clear reason?
Perhaps you're looking for conforming:
2 a: to be obedient or compliant—usually used with to <conform to another's wishes>
b: to act in accordance with prevailing standards or customs <the pressure to conform>
Or conformity as a noun:
3: action in accordance with some specified standard or authority <conformity to social custom>
A person who routinely conforms to established standards or authorities, without questioning those standards or authorities, would be called a conformist, and adopts a stance of conformism:
1. a person who conforms, especially unquestioningly, to the usual practices or standards of a group, society, etc.
Words to describe that behavior? I can't think of anything other than "bandwagoning" and "bandwagon effect."
Words that describe a person who engages in such behavior are much more plentiful though, and include "lemming," "sheep," or "trendster." "Yes man" might be appropriate, depending on what exactly you're going for and "fashion victim" is perfect for certain contexts.
There is a term to describe exactly this type of impressionable personality: sheeple - a linguistic blend of sheep and people.
sheeple (ˈʃiːp ə l)
— n informal people who tend to follow the majority in matters of opinion, taste, etc
[C20: from sheep + people ]
The phenomenon is probably best captured by Orwell's term groupthink.
1. the practice of approaching problems or issues as matters that are best dealt with by consensus of a group rather than by individuals acting independently; conformity.
2. the lack of individual creativity, or of a sense of personal responsibility, that is sometimes characteristic of group interaction.
http://dictionary.reverso.net/english-definition/lemming-like - definition #2
a member of any large group following an unthinking course towards mass destruction
In the animal kingdom, a lemming is a rodent known for periodic mass migrations that occasionally end in drowning.
Herd mentality describes such behavior of people esp., to the point of leaving out all reason.
Herd mentality (WP)
Herd mentality, or mob mentality, describes how people are influenced by their peers to adopt certain behaviors, follow trends, and/or purchase items. Examples of the herd mentality include stock market trends, superstition, home décor, etc. Social psychologists study the related topics of group intelligence, crowd wisdom, and decentralized decision making. mob mentality; flocking; mass hysteria.
Sequacious is the answer that seems missing here. I thing this would be the best word to describe this.
Assuming that the crowd itself is following someone (a "bellwether" or "lead dog" or "trendsetter" or "trailblazer" or "leader" of some other type), then the person who is doing what everyone else in the crowd is doing is simply a "follower," as in Ray Davies' fine song, "A Dedicated Follower of Fashion."
I don't know the precise context in which you wish to use it, but in a political setting, "fence-sitter" fits the description. They are on the fence until they know which side to stake and the choice is not based upon anything other than the popular position. While many would say "uncommitted" and it is a definition found on-line, in the political sense a vote must be made the fence-sitter just waits until the popular position is clear. It may sometimes mean a politician waiting for the "money" position to push him nefariously into agreement, but usually means the consesus of the populace. The fence-sitter simply waits for others to make his choice. Oh yeah add in "hipster:" the non-conforming conformer. ;P
You may be thinking of "affectation," as in definition 1. Though it differs slightly in its connotation, affectation has to do with pretense and attempting to project a particular appearance or generate a particular perception. Many people do things affectedly so that they can be perceived as being a part of the mainstream or "going along with the crowd," even if they don't genuinely understand what they're doing.
Definition of echo (n)
ech·o [ ékō ]
1. repeated sound: the repetition of a sound caused by the reflection of sound waves from a surface 2.something similar: something that repeats, imitates, or is reminiscent of something else 3. returned signal: the signal reflected by an object struck by a radar transmission, or the image of this on a radar screen
The man's ideas echoed those of the majority. Hers echo the crowd she entertains.
Okay minus vote. WHat was wrong with my answer?