It's kind of complicated why the distribution changed, we actually didn't change it and it fell into the state it was in at the beginning of this episode.
We have two MMR's that make up your whole singular MMR. Encounter and Win/Loss. Encounter is weighted more heavy in low ranks, and up to Episode 3 we didn't have any encounter for Immortal+. We saw a benefit in improving match making quality, and team fairness, by turning on encounter for Immortal+. To do this we had to adjust how much encounter was weighted for all ranks, because of this change it made everyones MMR shift down. This shift threw off our ranked targets and then made the community sit lower in rank than intended.
To answer the post, we've already made the rank target changes and have been adjusting slowly so it's not a huge jump and feels awkward. Distribution is in a good place now. That being said, every MMR system has issues with inflation/deflation, so you need to regularly watch this space and adjust. I'm assuming this won't be the last time we will have to adjust ranked distribution.