Good point and it's not cut and dried but the character of his output had changed to the extent that it was not published in technical journals as it had been:

>Relatively little of his work after 1960 was published by the conventional route of the learned journal, circulating initially in duplicated volumes of seminar notes; his influence was to a considerable extent personal. His influence spilled over into many other branches of mathematics, for example the contemporary theory of D-modules. (It also provoked adverse reactions, with many mathematicians seeking out more concrete areas and problems.)

My guess would be that he was working on himself rather than on the mathematics. Mathematics was merely the means and the medium. But I haven't read the material...!