As underwriter remember it'll be Citigroup's top priority to drive the price up after the 19th.
Citadel is selling all the shares they still had to Citigroup in an underwriting deal. That means, on the 19th, Citigroup will pay Citadel the prevailing market price (meaning the lower it gets before the 19th the lower the price Citigroup pays), but after the 19th the higher the share price the more profit Citigroup makes. Now, does anybody here doubt that Citigroup probably has a few completely legal, but ....you know...ways of driving the price up? After the 19th when Citigroup acquires their shares it'll be in their complete interest to drive the price up as high as they can. The the lower it goes before the 19th, and the higher it goes after the 19th both increase Citigroup's profit.