What was the immediate market trigger of the Panic of 1873 in Europe?

The immediate European market trigger of the Panic of 1873 was the Vienna Stock Exchange crash.

On May 9, 1873, heavy selling in Vienna followed a period of speculative investment and rapidly expanding credit. The collapse, known as the Gründerkrach, undermined confidence and helped spread financial distress through Europe and eventually to the United States.

The panic was connected to wider problems, including overbuilding, excessive railway speculation, weak bank balance sheets, and the end of a postwar investment boom. In the United States, the failure of the banking firm Jay Cooke & Company in September 1873 helped intensify the crisis.

The event is sometimes reduced to the American railroad panic, but the first major shock in the broader international crisis was the Vienna exchange crash. That distinction makes the Vienna Stock Exchange the specific answer.