I can only guess about all this of course, but "compensation" requires "withholding" and I expect that's what you're seeing. The compensation appears as a "cashless" transaction - you were "paid" in stock. But the withholding has to involve cash and that needs to come from somewhere and the most obvious place for that cash to originate is as additional withholding from your payroll check.
Withholding is really not "taxation." You don't know, ultimately, what your tax is until you complete your income tax return. Withholding is just an estimated payment against that ultimate tax liability and if too much is withheld then you get a refund and if too little is withheld you owe. You got some compensation, some taxes need to be withheld, your paycheck is the source of the withholding. The fact that you were paid in stock instead of cash changes nothing.
