In ʿalāma yalūmunī, the underlying form is ʿalā mā. When interrogative mā follows a preposition, its alif is typically dropped, as in bima, lima, fīma.
The question is not merely an attempt to obtain a reason. The speaker claims that no adequate reason for blame exists. The same construction therefore works on two levels: grammatically it is interrogation, and rhetorically it challenges the legitimacy of the reproach.