When multiple single females notice the advances of (at least 2) other females upon a male friend, a competition among the females for the male's attention/affection may erupt, and the male, sometimes unwittingly, gains a Harem.
The instance wherein a female realises she must compete with other females in order to obtain the role of being the most important female in a male's life.
When a single female that is friends with or interested in a single male notices that he has begun attracting the attention of other females, she becomes defensive/assertive and feels forced to act in order to keep from losing her significance in his life, or his significance in her life.
See: Twat Block, Alpha Female
see also: Reverse Harem, Reverse Harem Effect