This is part of the Sunnah. If I am not mistaken, the Prophet salla Allahu alaihi wa sallam used to call Aisha, may Allah be pleased with her Umm Abdullah. He even used to call children in such a fashion

A sister is asking most people use umm Ahmad, umm Rana , such name, but Prophet Mohammed is most time been addressed in his name instead of Abu Qasim ,and khadijah his wife is not addressed as Umm Qasim in most case ,is this sunnah? and why?