Rainbow Dash is a Chaotic Evil barbarian, that is the point, Chaotic Evil, why she needed to make the Shadowbolts become aggressive to attack? Especially in a situation where the others couldn't even SEE(in-game, of course) what she was doing?
Her player wanted to prove the others wrong. They basically said she was the worst choice for the Leave Your Quest Test because out of the whole group, she was the worst at actually roleplaying out an encounter. She wanted to demonstrate that She Really Can Actinvoked. Just flat-out charging the Shadowbolts wouldn't have shown the others she can do more than Attack! Attack! Attack!. Goading them into giving her a reason to go aggro on them, however…
Also previously, she'd readied an attack for "the first sign of hostility" that never arrived. If she could get the Shadowbolts to show a sign of hostility, she could trigger her readied action and automatically beat the Shadowbolts for initiative.