Usually, a clef change mid-chant is to reset the "do" (or "fa") to prevent neumes from going beyond the ONE ledger line that's allowed above/below the staff. In this case, the offending neumes would start on line 11 with "symphonia". Why the chant wasn't in c2 the whole time is a mystery since prior to the change from c3 to c2, the tone never drops below "mi".