The Medicare premium is based upon your federal Adjusted Gross Income plus your tax-exempt income. Since there was the income from the 401(k) entered on your tax return the AGI increased.
See - <a rel="nofollow" target="_blank" href="
https://www.ssa.gov/OP_Home/cfr20/418/418-1010.htm">https://www.ssa.gov/OP_Home/cfr20/418/418-1010.h...>
(6) Modified adjusted gross income is your adjusted gross income as defined by the Internal Revenue Code, plus the following forms of tax-exempt income:
(i) Tax-exempt interest income;
(ii) Income from United States savings bonds used to pay higher education tuition and fees;
(iii) Foreign earned income;
(iv) Income derived from sources within Guam, American Samoa, or the Northern Mariana Islands; and
(v) Income from sources within Puerto Rico.
