As a routine procedure, it seems that the underwriters typically get to buy an additional number of shares from the company offering stock to the public.
"Investment bankers typically sell 15% more shares in an IPO than they actually have. ... Included in every IPO deal is an agreement that gives underwriters the ability to buy more stock from the company at a slight discount to the IPO price. So if the price rises after the offering, the underwriters can buy the shares from the company that they have promised to other investors, but don't actually have, and book a small profit. ... In effect, the underwriters were short the stock. ..."
So far, so good. Underwriters, in effect, have a call option on the stock.