When you are successful, as the Houston Astros have been over the last decade, you are going to have some staff turnover as other teams lure talent away with promotions/more money. Unfortunately, the sign-stealing scandal fallout didn't help, which resulted in the loss of Mike Fast, among others, but a large swath of Houston's losses came down simply to completely normal turnover. However, this offseason has already been particularly busy when it comes to departures.
The Astros have already overhauled their coaching staff, with multiple former key coaches being shown the door. Houston also let their head trainer (a good change) as well as an assistant GM go as the team tries to reorganize and get back to their winning ways.