Irrelevant, lol was?
Humans base their actions upon a priori knowledge; if there is none, they have 50/50 chances of choosing between two unknown choices.
Because stockbrokers have apriori knowledge that by following the market trend you can minimize your losses. If against all expectations firm A's stock is rising in price, others keep buying, then heck yeah you join and buy a share now, assuming according to all your currently known facts that the prise will keep rising for now, and that is all that matters. Stockbrokers operate on the short run; good investors on the long run.
That is why for example Warren Buffet has had maybe the same stocks in his holdings for half a century, compared to modern investment banks trading billions of shares every second.
Therefore, following the herd is very rational, if you have had past experience that it works well financially. We humans learn everything by starting to mimic those people that are around us from the moment we are born, collecting experience; so that we later can base our choices upon the experience gained.
Objective rationality? But there is no such thing. We are humans, all our opinions are subjective. There is no 'objective' opinion, whether someone's action is rational or not. I once again restate that the concept of irrationality is a mere OPINION.
When talking about humans, their actions and preferences, please throw objectivity out of the window.
The market is based upon voluntarily participation, you produce good/service A, and trade it for service/good B with someone else. No one forces you to join the market; it just happens to be very beneficial upon everyone participating in the said free exchange of goods and services. People make rational decisions in producing their products and act rationally according to the best apriori experience they have; making rational choices that fit their needs and wishes best.
Buying products above market prices can be rational. Maybe you have imperfect information, such as you don't know the next kiosk sells the good A 10 cents cheaper? Maybe you are very thirsty and want to buy water 10 cents above market price? Maybe the seller is your friend and you decide to subsidize your friends or family? Maybe the cashier smiled at you, better than at the other cheaper store. You value the smile to be worth giving up 10 cents that could be otherwise spent.
All those above mentioned decisions have a good subjective rationale behind them. Human mind does not operate objectively; it is simply impossible.
The EGO, your mind, cannot make objective decisions. The optimum outcome, the one that maximizes your utility most at the cheapest cost, is always the rational choice.
I'm not sure if I can put this any clearer. Sorry that English is not my first language.