Well, leukocytosis can be caused by a wide variety of things and is generally found as a symptom, not an actual disease itself. Think about it like a runny nose. The runny nose itself isnt the problem, the cold is. Same thing with leukocytosis. It basically boils down to being an increase in white blood cell count.

Now, since you said that your dad busted his ribs, the undelying issue could have been an exposure of bone marrow, allowing leukocytes to stem into the bloodstream, or the associated hemorrhage . The other causes are generally more serious, including things like cancer, serious infection, etc. But considering your dad's condition, i find the former cause being the most likely. I'm sure he'll be well monitored and assuming there are no additional underlying causes, there shouldnt be much to worry about.