Most Helpful Girl
It could be because one of them likes you. It could have been because they had an amusing conversation previously with you in the center. It might have been that they'd had a conversation about a word, phrase, or line that people say in a previous conversation that you kept saying. It could have been they had a running joke they couldn't forget and kept laughing about something that had nothing to do with you. It could be that they had the giggles in the first place and couldn't help laughing at anything.
Honestly? There's no telling. Don't take it personally and just go with it. Girls giggle, particularly with their best friends/sisters.