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.

It is said by some writers that although Caesar defended himself against the rest and darted this way and that and cried aloud, when he saw that Brutus had drawn his dagger, he pulled his toga down over his head and sank, either by chance or because pushed there by his murderers, against the pedestal on which the statue of Pompey stood.