He cites family as the driving factor:
"The drain of being away from my family and other business interests proved to be too much and I have decided to resign from my role as Tigers hitting coach,"
I'm sure this is a situation in which the Tigers asked him to resign and did not want to resort to firing him. Obviously the bats weren't there all year and the organization would like to remedy that in any way possible. Good luck to Don in the future, and thanks for the hard work.