It’s not unusual for any individual to want to better themselves. In fact, you kinda hope that each and every individual has a fire burning deep within them to always be striving for personal improvement.
Life within sports is
A - similar in that, of course, there is an expectation to always be looking to better yourself.
B - even more likely to see this self improvement desire surface - with the very public nature of one’s employer and their respective abilities (if you are good at Tranmere Rovers, you want to move to Manchester (U or C), if you are going well in Cleveland, you (might) want to move to Miami where the Heat is stronger (you assume)).
So to see a coach move on to a new opportunity is ‘not an unusual occurrence’. Players do it all the time.