Several questions on this site relate to a situation where someone gets several offers simultaneously: for example, see How to choose between multiple math postdocs offers?. But how does this situation arise? In my (limited) experience, offers do not come simultaneously, and need to be replied to almost immediately (within a week or so). For example, position A could have the application deadline 1 February, have interviews mid-February and announcements mid-March, while position B could have everything one month later. To have two open offers at the same time seems very unlikely in a specialised field where positions are not open on a weekly basis. Then how does this happen?
In mathematics, the answer is easy: most American math postdocs are selected on about the same schedule. There are some variations of 4 to 6 weeks, but that still leaves plenty of room for second round offers from an early school to coincide with first round offers from schools that run later.
Further, there's been a tendency for schools that tend to compete over people to try to race each other to making offers, so the number of simultaneous offers ends up being higher than the number of schools would suggest.
(Also, a one week deadline to respond seems a bit short. What I've seen is more like two, with the ability to ask for an extra week or so which is usually granted if there's a chance of another offer.)
(Just for clarity, by most I mean more than half; there's a big early cluster and then additional positions being considered for months.)
In faculty hiring, it's not uncommon for the following sequence of events to happen.
- Candidate interviews at University A and University B
- Sometime later, University A starts making noises about making an offer (usually over email and on phone)
- Candidate puts out feelers to University B, hinting that if they were thinking of making an offer, now might be a good time.
- University B makes offer (email/phone) to candidate
- Much negotiating merriment ensues.
- Profit (for candidate at least)
Sometimes, there might even be more than two players involved in the bidding.
In short, the candidate can trigger multiple offers if they play their cards right. Or it can happen by fortuitous timing. But the approach described above is quite common.
In American sociology, hiring usually happens during the fall and winter months (although the 2nd tier market goes on to the Spring, which is a bit different from the fall/winter markets).
During these months, candidates (PhD Candidates, postdocs, VAPs, lecturers) send out applications for academic positions, usually in large numbers due to the high level of uncertainty and cut throat nature of the market-- Most positions get around 200+ applications (one postdoc competition I applied to last year was interdisciplinary social science and received 780 applications!).
I applied to about 30 jobs this season, which number is actually considered pretty low (I am in a pretty niche field) and I personally know other people who have applied to nearly 100 positions. The fact is, candidates have limited information regarding the hiring (what the department is "actually" looking for- because many things are not noted in the vague job descriptions), and it is believed that, getting an interview, is not only the workings of credentials and qualifications, but also largely due to "fluke." To up the chance, many people apply widely and in large numbers.
Anyways, many candidates do not get anything after months of putting in applications, and a few lucky ones can get multiple interviews, offers, and so forth (interestingly, I find that probability of getting interviews does not correlate too much with publication records either, except for the absolutely top tier market). It is completely possible to have multiple offers and when you do, it definitely gives you more bargaining power in negotiating.
I would think they happen mostly by having a relationship with the person doing the hiring. That means that the offer isn't made through a tight bureaucratic process but can be more flexible.
Many positions are never publically announced and the only way to access them is through networking. Some position will even be created to be able to hire a specific person.
Do you believe in the "element of luck?" Many people say that if you have this, you'll be able to achieve your life's goals in easier means as compared to others. And relating this to your question, those who receive simultaneous offers are probably "lucky" at that particular point in time. Unfortunate are those who really work hard but are not given such opportunity. Anyway, according to successful people especially those involved in real estate business, "persistence pays" - so never stop chasing what you truly deserve!