In short, it's a bad sign.

I glanced at the S-1. Professional money managers are offloading stock, perhaps due to a lockup ending which now allows them to sell. Some are selling some of their FLYY stock, others are selling all of it. I assume this was bondholders who got converted to stock in the bankruptcy. They are almost certainly selling at a loss; perhaps for tax purposes, perhaps because they don't see FLYY staying out of bankruptcy. Either way, they're selling because FLYY stock is dead money to them and they can get a better return on capital elsewhere.

Spirit will see no money on this sale. In fact, it is costing Spirit because they have to pay some of the filing costs. From the S-1:
We will bear all costs, expenses and fees in connection with the registration of these securities, including with regard to compliance with state securities or “blue sky” laws. The Selling Stockholders will bear all commissions and discounts, if any, attributable to their sale of Common Stock. See “Plan of Distribution.”