Knew someone who owned a String instrument shop. As I walked in he was just breaking a string on a cello - the second one he had tried to fit. I looked at the thing, and the string holder thingie at the bottom end was much closer to the bridge than normal. I pointed this out, and he reset it to nearer the correct distance. No more strings broke while fitting. He was having a bad day - something to do with a divorce...

Apparently some cello strings are hardened, and left annealed at the end where the tuning pegs are. With the different length, the hardened part was being placed on the nut at the top of the instrument - leading it to snap as it was bent slightly.

Robin