why would this happen

You have multiple issues there. First, the ground strap is missing and the braided hose is providing a ground path between the engine and the frame. That gives the sign that there is a problem between the hose and battery +, and that's why it got hot.

Second, and more important, the hose got "hot" which is an indicator of a direct short to the battery. Since the hose got hot between the engine and the frame, it tells you that there is a short from battery + to the engine that powered the issue above. When everything is set up properly the ground strap should just be protecting the system and is should not carry signigicant current.

Better get out the meter and check all of the battery + wiring at the engine to see where the short circuit connection is located or your problem will just follow the path of least resistance and could easily start a fire.