1. Stripe has to pay $3.5B in taxes. This is unrelated to employee stock.
2. Lots of long term employees have expiring options, and if they exercised them they would face a massive tax bill.
To solve both 1 & 2 Strips is doing an additional raise of $2.3B from private investors which will (1) give them money to pay that tax this quarter and (2) let employees exercise their options and sell shares to the same investors ($600M worth) – so basically a buyback event.