Objects not stored in their dedicated area in the cockpit may fall and cause hazards such as damage the equipment or accidentally operate controls or pushbuttons.
There was a story doing the rounds in the late 90s about a 747-400 crew doing the 'Annual Technical Questionnaire' whilst in the cruise. (Not the one that I helped to write, I'm happy to say.)
In the days before iPad 'search functions', the only recourse was to open the manuals and look up the answers. After several questions, there were five or six manuals open and lying around on the aft pedestal. A particularly obtuse question required the opening of yet another manual which was then balanced on the glareshield. Some unfortunate turbulence had the effect of tipping said manual and resulted in it falling onto the Fuel Control Switches. The sharp plastic edge of the manual's cover caught one of the switches and forced it over the safety and into the 'OFF' position! The story doesn't relate how they completed the subsequent ASR!