It is based on the women in the film Fatal attraction. In the film a married man has a fling with another woman. She falls in love with him but he won't leave his wife so she gets obsessed and boils his daughter's bunny.
I am not saying that obsession is good or indeed boiling bunnies but I can't help feeling; man treats two women badly and gets away with it as the spurned women gets the derogatory label as bunny boiler. It also advocates treating women like shit and perpetuates the belief that emotion=dangerous and bad, divorce from emotion= good.
It is also used flippantly.
E.g:
Me: I really miss my ex but I am very angry about the way he treated me.
Other: don't bv esuch a bunny boiler- move on!
The above is just an example.