It is common for boots to sag at the ankle after they are worn and broken in. If, however, the leather sags so much that it causes dimples (creases) inward toward the back of the ankle and rubs, then sores may develop.
Leather sags as the fibers in the cellulose material that compose it loose connectivity as the fibers break down with time. Simply, there isn't much left holding the boot shaft up, so it falls down.
It is an “old wives tale” that you can stiffen leather by soaking it in water and letting it dry. Yes, for a very short period, it will appear as if the leather is more stiff. But shortly after wearing the boots again, the leather fibers that reconnected when exposed to water will break again, and the boot will return to its usual sagging state.
One may think that one can add starch to the water in which boots may be soaked. Yes, you can do that and yes, starch is cellulose. However, all that ends up happening is introduction of loose cellulose (starch) fibers which do not remain connected when the boot dries.
The best way to deal with sagging boots is to use a professional “boot shaper” which is available on the specialty market, or less expensively, stuff the boots with brown kraft (“butcher”) paper firmly. Do not wet the boots. Stand the boots up in a closet for a week. Then see if it helps.
Likely, though, once boots sag and loose shape, they will remain that way. Either accept it, or dispose of the boots by selling them, giving them away, or discarding them.