I am not directly affiliated with Starbuster (as of writing), I am merely a very interested fan. --
Is Cassie being a tease to get the attention of someone watching from out of view? Or is she just making some noise to see how everyone will react? Imitating some video she found on the Internet?
Did she botch the landing while swinging through a crowded carnival and is embarassed that everyone saw her land in such a compromising position? Or maybe she's just feeling frisky and decided to have a good time with the first soft thing she had in reach?
Personally, I like to think Cassie got this particular balloon from some kind of mod chip found on the field during a mission near an abandoned elastics research facility, brought it back to Petrov, and then started jumping around and bouncing on it to tease Sally (who finds Cassie's antics harmless and funny) and Jun (who hates the noise and is keenly aware that if Cassie hurts herself doing stupid things with the balloon, it'll be her responsibility to care for her). Early into her mischief she finds the phsyical contact soothing. Then pleasing. Then... Oh. OH. She has second thoughts, afraid to get busted by either of the sisters for being Making Things Uncomfortably Weird. But the more she feels it press against her, the less stopping feels like the right answer. She almost wants Sally to notice. But Jun is much more likely to recognize the signs. The mod itself being a companion piece to the balloon-making mod from https://www.deviantart.com/revelromp/art/Month-of-Cassie-20-Pump-It-Up-858739104 intended to dynamically adjust the shape, color, and elastic properties of any given balloon.
One of the fun things about player characters? Everyone plays the character a little bit differently. And while there's certain personality traits and defining characteristics that don't change, there's lots of room for each player to have their own particular flavor of Cassie.