What caused "Operation Valkyrie" to happen?
Operation Valkyrie did not ever actually take place. This was a plan that was supposed to be implemented after the death of Adolf Hitler. However, Hitler did not die (at that time) and so the operation was not actually launched.
Operation Valkyrie was to be ordered by the plotters who were going to kill Hitler. The plan entailed arresting Nazi leaders and taking control of the country. It was to begin after the plot to kill Hitler was successfully carried out. However, the attempt to assassinate Hitler did not succeed and the plan was never carried out.