Why may teachers change during the course?

Well, there is not one single answer for that, depends on each case, but reasons can be many.

Teachers may need to slow down or take a break after a long period of hard work, they may need time to be with their families or to take care of other personal commitments, they may fall sick or go on vacation...

From an organisational or academic point of view, since there are usually new students or groups starting every week, since it is beneficial to be exposed to multiple accents to improve intelligibility and since some of our team have other tasks besides teaching, it may also be necessary to reorganise things from time to time.

In any case, decisions are not arbitrary, they always respond to a necessity.

Since programmes, contents and materials are planned by the academic team, these will not be affected.

Obviously, if doing a private course, students can choose at the time of booking which teacher they want to have classes with (availability will most likely be limited, though).