As PP say, the Y6 teachr may have requested the change, but I was coming on to say what @LikeaSnowflake said.
Sometimes it emerges that because a Y6 (or Y2) teacher has been in that year group for a good while and is very effective there, other teachers in the key stage may not be supporting the children's progress sufficiently through the previous years. It doesn't 'show' outside of school because the teacher of the oldest class makes up the difference, but it means the children (and the Y6 teacher and any TAs) can have a last year that is tougher than it needs to be. The other teachers, who have never taught in Y6 because that is Mrs. X's domain, may not fully recognise the expectations and Mrs X may moan but get on with it.
When Mrs X moves to Y3, she knows the key things children need as a bedrock and foundation to build on through KS2. Mr Y moves into Y6 and learns the urgency to secure particular skills and knowledge, but takes with him his understanding of how the underpinning knowledge is acquired. Win win! (Although it is a massive shock to the system moving across or between key stages if you have been in one year for a few years, and it does take a while to adjust your speed of teaching and expectations).
The other possibility is that there has been a new HT in your school in the past couple of years. As PP has said, it is more normal for teachers to move year groups regularly, so a new HT may have inherited a tricky and less than ideal situation with Mrs X as the Y6 teacher whose position may not be questioned. The new HT may have acted immediately, or they may have observed for a year or two, but now is the time for a shake up.
Whatever the reason, it is not a demotion!