An anonymous poster responded to something I had written to say that regulations of the Office of Management and Budget prohibit federal agency heads from lobbying for appropriations in excess of what is contained in the President's recommended budget. This is true and an important fact for agencies other than Social Security.
However, the Social Security Administration is in a unique position. The Social Security Independence Act now only allows, but requires Social Security to submit its own budget request directly to Congress, which means that the Commissioner of Social Security is free to lobby Congress for whatever budget he feels that his agency needs. Indeed, if the Commissioner of Social Security is unwilling to do so, there is little point in the Social Security Administration being an independent agency.
In refusing to advocate for his own agency's budget, Michael Astrue is making the case that the "independence" of the Social Security Administration is a sham and that in the next administration Social Security should either be returned to the Department of Health and Human Services or made a cabinet level department. Either of these would put Michael Astrue out of a job, which might not displease a newly elected Democratic president.