What cause a conrod to break?

My car a Opel Corsa 1.6i 2005 Brake down I notice exesive smoke from the exhaust and pull over switch the car off open the bonnet and touch the engine it was not hot when I try to start the car again it did not start the car was towed in, the diagnosis was the head gasket The head was send for machining and after a astronomical account I was happy to get my car back after 3 weeks My first long distance that I do after the head was done the car brake down again They say two conrods broke and 2 holes in the block Now I need a new engine The Garage that worked on the car claim that they worked on the top and not the bottom of the engine they refuse to take responsibility What cause a conrod to brake?