How did Charles even get the note from Jill? Did he purposely wait until he was driving to write his? How did he intend to send her his reply, was he perhaps driving over to her place to hand it over in person? Then why write it in the first place, and not just tell her!
It’s just a silly anachronistic texting while driving joke. Of course a horse wouldn’t ride off a bridge in real life when someone is writing a letter. But a car would if someone is texting.
It's not because the driver is texting (distracted), but for some reason it got scared or confused, and because it lacked the control of the carriage driver, it took a wrong turn.
They are not without intelligence, but they are still stupid compared to humans. They are also easily scared, and when they are they behave erratically.
But I changed the text to omit that part completely. Because somehow it seems to offend people?