How do people get simultaneous offers? 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.)