Past practice has seen pretty much every president for the past 30 years or more has had a number of high level officials quit or be fired. It happens. If you look back to Obama he got rid of a few…Holder, Napolitano, Geitner, Sebelius for examples. I don’t know how Trump stacks up to past presidents, but I suspect it is higher than most. I can see reasons for some of the changes. He had many leftovers from the Obama era that were actively working against him so they had to go. Others may have had internal struggles with the administration.
I will say that moving department heads around is a pretty standard practice in the business world. Every company I have worked for has replaced or cycled top echelon positions fairly frequently…a big shake up every couple years at least. Maybe Trump is applying business strategies to how he runs his administration.