No one outside of gaijin knows for sure. The consensus is that “skill” or past match performance probably has no impact on it.
The matchmaker seems to just fill matches by whats available at the moment. The chance of getting uptiered/downtiered will be highly dependent on what vehicles around ±1 BR from you, that other players are waiting to use at the moment.
Changing servers or playing at different hours can give very different results on what BR range you end up joining.
There are somewhat persistent BR “blackholes” where there is a disproportionate high number of players trying to play something from a specific BR. Top tier is always one, and the usual spammed premiums often cause the same issue at lower BRs
The matchmaker tries its best to limit the top and bottom BR player numbers between both teams to be about the same, but sometimes you get very unfair matches, where both teams have very different average BRs. I had matches where my team didn’t have any top BR players for the match, while the other team had many.