How did Ginger die?

From the text, we can infer that Ginger died from overwork and abuse.

Just then her driver came up, and with a tug at her mouth backed her out of the line and drove off, leaving me very sad indeed.

A short time after this a cart with a dead horse in it passed our cab-stand. The head hung out of the cart-tail, the lifeless tongue was slowly dropping with blood; and the sunken eyes! but I can't speak of them, the sight was too dreadful. It was a chestnut horse with a long, thin neck. I saw a white streak down the forehead. I believe it was Ginger; I hoped it was, for then her troubles would be over. Oh! if men were more merciful they would shoot us before we came to such misery.