When Julius Caesar lay dying after receiving multiple stab wounds why did he try to cover his face with his robe? Out of decency? Out of shame? Out of sadness? Out of disgust? Out of disdain?
Goldsworthy (in his biography of Augustus) says it was one last act to preserve his dignity. Obviously this isn't a primary source and nobody but Caesar could really say. However given how extensively Goldsworthy has written about Caesar I'd be inclined to say that's the best guess we would have.