Elsa watches her stumble a little as she giggles, and the second guy moves closer to steady her.
That, if nothing else, is what convinces her to move.
As she gets closer, she can see little plastic shot glasses, once filled with jelly-vodka, now empty. The smell of weed fills her nose, and it just gets her moving faster because that means she's drunk and high and oh god what if something happens? Just the thought of Anna being hurt again makes Elsa want to cry. She wouldn't be able to live with herself if something happened.