The following contains spoilers for RWBY: Ice Queendom Episode 12, "Best Day Ever," now streaming on Crunchyroll.
RWBY: Ice Queendom has been a wild ride in expanding the lore of the main RWBY canon. The anime explored a parasitic new Grimm, introduced a new type of Hunter that specializes in capturing this type of Grimm, and had the main characters fight internal battles on a more personal level. Set between Vols. 1 and 2 of the main RWBY series, Ice Queendom sought to bridge a gap in Weiss Schnee's characterization by addressing her unhealed childhood trauma head on, and how that was impacting her relationships with her new friends and teammates.
After a long, arduous battle against the Nightmare Grimm possessing Weiss's body, the members of Team RWBY resume their everyday lives at the Beacon Academy, concluding with a new take on the iconic RWBY Vol. 2 episode "Best Day Ever." Though the new version of the iconic food fight replicates many memorable moments from the original episode, it also successfully integrates Weiss's character growth from the Ice Queendom storyline in an unexpected, yet logical way. While the original episode only implied who really started the food fight, Ice Queendom confirms it was Weiss’s idea after a pie was accidentally thrown at her face.
Weiss Started The Iconic Food Fight - And It Makes Perfect Sense
As unusual as it may seem that a rule-oriented girl like Weiss would ever think of starting a food fight, at the same time, the reveal makes sense within the context of the Ice Queendom storyline, and is consistent with the themes explored. The first detail established about Weiss in the first three episodes of Ice Queendom (that wasn't really touched upon in Vol. 1 of the original series) was how profoundly unhappy she was with her home life.
In Weiss's first episode, she's depicted as stoic when interacting with her family members, all of whom are invalidating of her fighting skills and her decision to attend the Beacon Academy in Vale to become a Huntress. The only people who validate her decision are her older sister Winter and their butler Klein Sieben, the latter of whom is more affectionate toward her. For fans familiar with the original RWBY series, Winter being supportive of Weiss's decision to leave Atlas for Vale is explained as her sympathizing with her little sister's desire to forge her own path in life, seeing that Winter did the exact same thing for herself.
Winter being supportive of Weiss also contextualizes why she idolizes her older sister as much as she does. Wanting to live up to Winter's expectations also informs those Weiss has for herself when she enrolls at Beacon Academy. Not only does she take herself far too seriously to the point of condescending her partner Ruby Rose and idolizing Pyrrha Nikos -- whom she sees as her equal -- but it also has her behaving in ways that's seeking validation from other adults. Specifically, she wants the school headmaster, Ozpin, to recognize her academic performance and her talents as a fighter, and be named team leader so Winter can be proud of her.
Weiss, of course, doesn't get what she wants, which ultimately leaves her vulnerable to the Nightmare Grimm that eventually possesses her body. When Weiss is left in a coma, the Nightmare Grimm constructs a hellish dreamscape built from her memories of childhood trauma and unresolved anger toward her father Jacques. Weiss's idolization of Winter manifests in the form of her wearing an Atlesian military uniform similar to the one her sister wears, while her inner child is kept locked up and hidden. When her inner child is allowed to run wild and free in her nightmare world, it reveals all the childhood desires Weiss was forced to suppress as a consequence of being emotionally abused.
Weiss Starting the Food Fight Finally Gave Her a Fun, Childlike Experience
Among Weiss's childhood desires include quality time with her family, being validated by her parents, and being able to do all the kinds of fun activities normal kids get to do. Fun activities like attending amusement parks and eating sweets, and being able to play outside with a group of friends. The fact she didn't get to do any of this as a child is why her decision to start the food fight in "Best Day Ever" is made more significant.
As a 17-year-old girl who's quickly approaching adulthood, this is one of the few opportunities Weiss has to do something truly fun and childish. Instead of being standoffish and reminding her friends of the school rules like she normally would, she uses the opportunity that a pie gets thrown in her face to let herself loose and do something she's always wanted to do: break the rules and be a little naughty. Get in trouble for something genuinely stupid like starting a food fight with her friends, as opposed to getting in trouble for displeasing her narcissistic father.
Since the whole point of RWBY: Ice Queendom is setting Weiss on the path to becoming a more authentic version of herself, her starting a food fight is the best way to end her character arc.