D4 to D3 you mean K4 to K3, right?
All of my experience with squish is that it's happened right after league promotions/demotions. So this before the season starts, and right after the last match has taken place and you get your end of season rewards.
Having it happen after the new season starts is totally new to me, and definitely not what I have seen with squish. Is it a high-mid only Kyber thing? As 100% I was squished out of A1 before the last 3v3 season started.
So most of my guild is caught in this K4 - A3 whirlpool. We seem to continually climbing to K4 and then a pretty heavy magnet pulls us back towards A3, rather than being pushed to K3, which you'd think (as the leagues are 'diamond' shape). So this must be because of the number of players ending up in A1 and being promoted to Kyber. So my thought is that the end of last season, this wasn't the case.
I dunno, I think we're both going in circles here and one of use isn't quite understand the other (probably me).