Should RSUs sold to cover tax be subject to capital gains tax if they were only sold to cover taxes in the first place? They never were before and I did not cash out.

I saw a capital gains tax of one way more than my return as ever been before and I’ve had RSUs for 10 years. I’m figuring it was a processing error because I didn’t get any of that money in my bank account except the usual difference. Is the difference subject to capital gains? If so, why is the tax 4x higher than the difference itself?