The Great King long ago ceased attempting to police the wastelands of his Border Marches, and these debatable lands fell into the hands of petty counts, retired generals, and warlords, who constructed and then abandoned many keeps ripe for plunder.
Rain slowly builds, thunder and lightning continue to roll in. Road muddies, horse/wagon getting stuck in the mud. Thunder strikes coinciding with a deep hole a horse just walked in, horse panics, breaks its leg (maybe just sprain?). Horse is decompacitated and the rain just went from pouring to an all out monsoon. Shelter needs to be found, horse needs to be taken care of, covered in mud, add the posibility of items being lost to the confusion with the wind and dealing with spooked animals.