Why is it, the friends of the girl you are talking to are keen on removing her from the interaction overall. Even when the girl is clearly interested and into it?

I find this happends quite often, or rather it did happen quite often in my first ever night out. I approach a group of girls with 2 or 3 of them in there little circle and I make a straight-line drive to the girl I am most interested in. I don't introduce myself to the group, instead I say some witty shit or something that grabs the attention of the girl I am interested in and basically try to physically remove her from the little social circle with subtle touches and what-not. I put my hands on her waist or her back, and she is enjoying everything. We get up close. the eye contact is strong and the conversation is fruitful. Then, her friends pull her away and say they need to use the washroom. WTF. Is her friends cockblocking her because they can't get any dick? Clearly she is enjoying the conversation and her time, why would you do that? Out of the 4 times this happened, 2 times I approached the girls trying to push her away and question them, like why do this, and no response. Other times I tell the girl to stay where she is, or tell her that her friends are boring or something. What can I do?

What to do in this situation, to lessen the chances of this happpening. And why does this happen?