What does it matter to know the 'why' of their bullying? What matters is your response to it. What is wrong is that you choose to keep going back to them and endure further bullying. You do not protect yourself. You do not set clear boundaries. You cannot control what other people do. What is in your control is what you do.

Because she knows she can get away with it and you fall into their trap. Over and over again. You go back to them. You initiate contact. By doing this, you are basically giving them permission to continue to abuse you.

Frankly, it's like you putting your hand in fire, your hand gets burned, and you go 'oh, why is the fire burning me, what is wrong with the fire?' What you need to learn is that this is the nature of fire - it burns. Bullying is the nature of those people whom you consider to be 'friends'. The important question is: WHY do you choose to keep putting your hand in the fire?