Earlier quoted context omitted.
Because they didn’t get options. Stripe was flirting with going public for so long that they gave RSUs and people were trading the shares on private secondary markets. Those secondary markets have dried up in the general macroeconomic environment, so now this practice is leaving people with their pants down, complete illiquidity.
If they didn't get options, tax would have had to be paid as each RSU vested if it was not paid with an 83b election. So again, I don't see why Stripe has a tax bill now.
Some of the RSU have upcoming expiration dates as required by IRS rule. To avoid those expiration dates Stripe wants to do something. That something probably involves a big tax withholding for Stripe that must be paid in cash.
Stripe doesn’t intend to go public soon so is seeking an alternate way to come up with that cash.