One answer is that when one is instrumentally incoherent one violates some independent requirement of reason.  That is, either one has some specific attitude (or pattern of attitudes) that one ought not to have or one lacks some specific attitude (or pattern of attitudes) that one ought to have.  The source of the problem, according to this answer, is not that when one is instrumentally incoherent, one violates some requirement of reason to be instrumentally coherent as such.  The source of the problem is instead that when one is instrumentally incoherent, then it is guaranteed that one’s having (or lacking) one attitude (or pattern of attitudes) violates some independent requirement of reason.  (See Kolodny 2007b, 2008a; Kiesewetter 2018, esp. Ch. 10; Lord 2018, esp. Ch. 2.)