It might sound weird but the 2 most likely ranks to be toxic are silver and plat.
This is because they are close to positions of importance.
Silver is better than bronze, so people coming out of bronze usually feel like they are finally getting good. So you have people that are cocky that they are out of bronze. Gold is the first rank where league really seems to acknowledge that you are good. Gold players are in the top 40 percent of best league players and get access to the awesome free skin and get access to the way better gold border.
Silver players feel like they deserve gold considering they are very close to it. It feels much more rewarding, it feels a lot better to show off to your friends.
Silver is the rating stuck between where a lot of players want to be and where people really don't want to be.
Plat kind of has the same problem as it is close to the rating that people REALLY want. The saltiness and competitiveness might even be worse there because of the:
- Smaller playerbase= more autofill
- Feeling better
- Really wanting that rank up.
TLDR: It is because it is one of the two worst ratings to be in from a perspective.