DISCLAIMER: I am NOT condoning any of their behavior, nor would I react in such a way, but in approximately 15-18 months, people will be paying me to play Devil's Advocate, so I like to take advantage of practice opportunities.
With that said....
I would like to ask what your relationship was like with your line sisters prior to coming out to them. Maybe you jumped the gun or maybe they felt deceived by your lack of honesty when it may have been warranted.
Even though the bonds of sisterhood may be strong, some people may still be faced with those barriers of not revealing all about themselves up front. Some people, especially women, need time to get to know you before they "tell you all their business." Contrary to popular belief, being "on line" with someone does NOT make you instant best friends. And though YOU may feel comfortable with these people, the feeling may not be reciprocal.
The converse may also be true. Depending on how long you have known your line sisters and the experiences that you may have shared, they may be reacting to the fact that you waited all this time to tell them. I know some of the experiences and information that my line sisters and I have shared from the day we met, and I would be HIGHLY pissed if one of them waited until NOW to tell me that they were a lesbian!

Also, as someone previously stated, they may be questioning your motives and integrity in that you decided to reserve coming out for AFTER you became a Delta.
None of us can state why they are choosing to put your business out like that. Maybe for some of the same reasons that sites like dontdatehim.com are popular. A person gets hurt or angry and doesn't know how to react. They need some other people the support and validate their anger. I also remember the person that I was in undergrad. Though I was mature and better raised, I can't necessarily say that I would have been above the behavior. With the right kind of support (which sounds to be campus-wide), I could see how easily I could have gotten swept up into keeping some mess going. You're dealing with 18, 19, 20 or so year old chicks. I know some grow azz 25+ year olds in LAW SCHOOL that pop off drama like high schoolers, so I put nothing past anyone.
Just live your life and do you.