reftex-sort-bibtex-matches

reftex-sort-bibtex-matches is a variable defined in `reftex-vars.el'.
Its value is

reverse-year

Documentation:
Sorting of the entries found in BibTeX databases by reftex-citation.
Possible values:
nil Do not sort entries.
'author Sort entries by author name.
'year Sort entries by increasing year.
'reverse-year Sort entries by decreasing year.

You can customize this variable.