This effect is called palatalization and affected not just the transformation of C to CH, but other involved other sound changes such as G to J.
As Greg Lee says the Joret line is important, the palatalization occurring south of the line. Though Franks were located in this area, it seems unlikely the Franks were the origin of the palatalization because their Germanic language featured hard consonants, so it would seem to be a change adopted by the Franks, not originated by them.
Many studies on palatalization have been done and these have identified the time of the change as occurring between 800 to 1000 A.D., the viking period, but the vikings cannot be responsible because the palatalization is not found in their languages.
A more interesting idea is that the transformation originated with the Veneti. The reason for thinking this is that the variant of Breton spoken by the Veneti, called Vannetai, strongly features palatalization, much more so than other dialects of Breton. Also, the Veneti are found elsewhere, such as in the Veneto area of Italy (northeast), and it likewise features palatalization in those areas. Thus, there is the possibility that the Veneti influenced somehow this sound change in southern Normandy.
Gvozdanović, Jadranka. "On the linguistic classification of Venetic." Journal of Language Relationship• Вопросы языкового родства 7 (2012): 33-46.
Gvozdanović, Jadranka. "Evaluating prehistoric and early historic linguistic contact." Historical Linguistics 2013: Selected papers from the 21st International Conference on Historical Linguistics, Oslo, 5-9 August 2013. Vol. 334. John Benjamins Publishing Company, 2015.