How do editors assess urgency of submissions?
There are some journals which try to guarantee expedite review and publication processes -- typically less than one month, even 1-2 weeks, for the review process -- for short papers which can have a significant impact on rapidly developing fields.

In physics, examples of such journals are Physical Review Letters (PRL) and Applied Physics Letters (APL). The word letters in the titles aptly recalls the above qualities. 

From the About section of PRL, we can read (emphasis mine):


  Authors gain high visibility, rapid publication, and broad dissemination of their work.


and


  
  If presenting a new technique, or methodology, it should play a pivotal role in future physics research, and make apparent its immediate consequences for physicists.
  


From the Criteria For Publication section of APL we can read:


  Timeliness: One of the questions on the reviewer's checklist is: "Is the paper especially important, interesting, and timely enough to warrant rapid publication in Applied Physics Letters?" A fraction of papers, which report new results but in a mature field, do not fulfill the condition of timeliness required by APL, and should be submitted for consideration as a Communication elsewhere.


Thus, for instance, one might not have much chance in publishing on PRL a work about classical mechanics, which is a well established and no longer rapidly developing field, but might have a much better chance in publishing an innovative work on, say, quantum point contacts.

If your submission was sent to such kind of journal, the editor, on the basis of their knowledge of the field, might have decided that your work would have not had an immediate impact on the field, and it would probably fit better in a journal with standard reviewing and publishing times.