Hand grab

Fiddling, between the new engine and the port-side engine bay wall, I'm interested to discover that there is a certain pattern of pressure on the palm of the right hand that creates a visceral panic reaction. Conscious control returns in a second; I know that the engine isn't bolted to anything and my hand cannot possibly be trapped, but what an instinct. In what murky past did one of my ancestors survive only by having this built in fear?