Sure thing! In all honesty, that was something that I heart from my mother. (She works in psychology, but she is not an academic. So I should probably retract the "well" in "well-researched") But a quick research turned up these two sources:

General info about Victims, Bullies, Victim-Bullies

https://www.apa.org/news/press/releases/2010/07/bully-victim

More on point: In-depth about who is more likely to become a victim

http://www.euro.who.int/__data/assets/pdf_file/0008/98783/E9...

I haven't read these thoroughly, but hopefully it's a place to start. I searched for "picking victims study." That seemed to come up with some hits. A search for "group dynamics abuse" was less helpful.

Targets who displayed vulnerable body language were more likely to report past histories of victimization, and psychopaths identified these individuals as being more vulnerable to future victimization.

https://www.workplacebullying.org/gait/

What Do Sexual Predators Look For When Grooming Victims?

https://www.oxygen.com/crime-time/jeffrey-epstein-what-do-se...

Short Circuiting the Victim Selection Process

http://www.protectivestrategies.com/victim-selection.html

Marked for Mayhem

https://www.psychologytoday.com/us/articles/200901/marked-ma...