You have Shimano STI shifters, right? I've seen this happen several times. Even me - had one break on my first set of 600 8s shifters. Replace your STI shifter cables on a regular basis. Annually or every other year is a good idea.
And I've never seen this happen with a Campagnolo Ergo system.
__________________
72 Frejus (for sale), Holdsworth Record (for sale), special CNC & Gitane Interclub / 74 Italvega NR (for sale) / c80 French / 82 Raleigh Intl MkII f&f (for sale)/ 83 Trek 620 (for sale)/ 84 Bruce Gordon Chinook (for sale)/ 85 Ron Cooper / 87 Centurion IM MV (for sale) / 03 Casati Dardo / 08 BF IRO / 09 Dogma FPX / 09 Giant TCX0 / 10 Vassago Fisticuff