I heard from old-timers that before the war many women kept Shabbos and were entitled to be buried in Shomer Shabbos sections while, sadly, most of their husbands and sons did not stand up to the incredibly difficult Nisayon of Shmiras Shabbos in those years and they were refused burial in Shomer Shabbos sections.
By the time the 1930s came around, cemeteries evolved into men’s and women’s sections originally being non-Shomer-Shabbos and Shomer-Shabbos sections.
People in our time just continued these policies of the old cemetaries. Many thinking that it is some sort of Tznius or Frumkeit when it is just a relic of a very sad chapter in Jewish American history.
This is easily disproven by the existence of much older cemeteries which have separate rows for men and women, and by the fact it is not separate sections, but separate rows. That is:
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