My theory: White Diamond /Pink Diamond rivalry. White shattered Pink and Yellow had her suspicions. Yellow was either very close to White or looked up to her like an Idol. Which is why she reacted in that way
Usually I am not the biggest fan of the moustache as a branding element, as it is very common to use it currently but I have to say that I do really like how the branding comes together here, especially the colours are working great imo /// m.