When I switched positions about a year ago, this is how I explained it to the teacher who was replacing me:
My lessons are always evolving and I've never taught a class the exact same way twice. There are many factors that go into this decision but some of them are:
1- Unique student needs
2 - Improving student engagement
3 - Implementing new ideas
4 - Editing things that aren't working
With that being said, I always start with what I've done in the past and use that as a base (unless the curriculum has changed).