bibtex-maintain-sorted-entries is a variable defined in `bibtex.el'.
Its value is

  • This variable is safe as a file local variable if its value
    satisfies the predicate which is a byte-compiled expression.

If non-nil, BibTeX mode maintains all entries in sorted order.
Allowed non-nil values are:
plain or t All entries are sorted alphabetically.
crossref All entries are sorted alphabetically unless an entry has a
crossref field. These crossrefed entries are placed in
alphabetical order immediately preceding the main entry.
entry-class The entries are divided into classes according to their
entry type, see `bibtex-sort-entry-class'. Within each class
the entries are sorted alphabetically.
See also `bibtex-sort-ignore-string-entries'.

You can customize this variable.