I have been reading Madigan's excellent new survey of medieval Christianity and one rather off-hand statement struck me as fascinating:
The wife of Clovis II (r. 639–657) was a woman from the lower orders of Saxon society in Britain; it was customary for men from the Merovingian line to ignore matters of “social class” when selecting wives.
Madigan, Medieval Christianity: A New History, (Yale University Press, 2015), p. 240.
This particular woman was Balthild (d. 680). Why would a Merovingian monarch choose a wife from among the 'lower social class' - especially of another gens? Was there a particular political aspect to this union? Why might Madigan note this as particular feature of Merovingian kingship and, if correct, why did it not continue into the Carolingian or Ottonian lineages?
Fouracre in Late Merovingian France: History and Hagiography, 640-720 (starting at p.97) suggests that Balthild may have not been of 'low birth', reading the Liber Historiae Francorum to say that she came from Saxon nobility.
Reading between the lines here - as Fouracre doesn't come right out and say it - is that there is a matter of political hagiography going on here with casting Balthild as a slave bought from Saxons and rising to queen, in particular as the main source of info on Balthild, Vita Domnae Balthildis, sets her story against stories from Kings and Psalms about 'raising a poor man out of the dunghill'. But how did a slave, a 'cup bearer' come to Clovis? It appears that she was actually a 'gift' from within the Frankish political factions, from a mayor of the Palace, Erchinoald, to Clovis. Erchinoald was Balthild's master and gave her to Clovis. Moreover, Fouracre points out that at exact time Balthild was married off to Clovis there were machinations between Erchinoald and the 'English' (Saxons) which had influenced this choice, one which helped hime 'consolidate his position' with the English and the Franks.
So, hagiography aside, I'm not sure that this was entirely about 'marrying down', Madigan may be emphasizing the wrong thing here.