This is an exerpt from the Pirastro site, they have hints on why strings may break. There are some things I had never heard of like loosening the strings to allow them to regenerate (gut).
http://www.pirastro.com/btbreakage.html
Gut wound strings:
1. String gets damaged by sharp edges of bridge or upper nut that bands cut the gut core, for instance when d-bass strings are too long for individual instrument
Pure gut strings:
1. Gut gets damaged by sharp edges of bridge or upper nut groove , that gut diameter will be finally too thin for the necessary tension and breaks
1. all parts in direct contact with string have to be well rounded and
2. should be coated with a soft pencil to let the string slide.
3. don’t twist string cross-wise on peg shaft.
4. for string with loop end use protected fine tuners to avoid metal sharpness (E-string problem!)
5. for string with ball end use fine tuner in right size (too small hooks are string killers)
6. gut and gut strings are living strings: tuning over the pitch and back down into pitch lowers tension between peg and upper nut. Lower tension ¼ tone when on tour or not playing. The gut will recreate and be fresh again for the next audience.
7. gut or gut wound strings could break when brought and kept tuned into dry air. (gut will loose it’s normal humidity and shrink:tension increases)
