The ...abus dative/ablative plural is a rare feature of the first declension that can in exceptional cases be traced back at least to classical Latin. For example, you will find Cicero saying (Pro C. Rabirio perduellionis reo):
ab Iove Optimo Maximo ceterisque dis deabusque immortalibus [...] pacem ac veniam peto
Clearly in this case Cicero uses the form to explicitly call upon goddesses and could not very well have said dis disque. (He could have said dis utrius sexus or something to that effect, but how does that sound?)
So medieval Christian authors arguably did not invent this form out of thin air, but they took a rare form and applied it to many new words. Other forms were famulabus (for example in the Oratio pro his, qui in coemeterio requiescunt) and monachabus. While these make sense to explicitly mention women (Who would have thought medieval Christians were pioneers of gender-inclusive language?), the same cannot be said of ecclesiabus, villabus and other forms, which are also found.