Most likely, they didn't get return invites to their favorites. Rather than looking at each day of recruitment with new eyes, and giving sororities that were interested in them a chance and a good look, they chose to drop. I hate to see that happen.
UVa Panhel Recruitment is a tough nut to crack and has been so since at least the 1970s. Rushees definitely need to keep an open mind even if they're a legacy multiple times over to a specific group.
As an offside comment, I do find it interesting that some parents will be critical of the legacy group not choosing the legacy daughter but have no problem with a different legacy daughter cutting the legacy group at a different school.
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....but some are more equal than others.