If Box 5 contains income not subject to federal tax, why did payable federal income taxes spike when unvested retirement benefits (not 401K) were included with wages?

I turned 65 in 2018.  My company added the value of my unvested retirement benefits (stocks and long-term cash incentives, but not 401K) to Box 5 of my W2 form.  This figure was on top of my regular Medicare wages and tips.  The result was to spike our federal taxes due.  Is this a glitch with TurboTax Premier (which I'm using) or is this supposed to happen:  Are our federal income taxes on unvested retirement benefits supposed to be collected at this time?