I know you're not going to like this but here goes: when you demote your ticket holders number gets cut by 25%.
Now, for most teams, this gets them right down in the line of the average amount of the ticket holders of the lower division. However, there are outliers, like your team, who had rather high number to start with and the 25% cut still put you quite higher than the D3 average.
Now, during the season, your team performed well, but the number it tended toward to (an imaginary upper limit for D3) was still lower than the initial number you had! That's why you had a drop, your initial number of ticket holders *should* have been lower to start with.
Why is this so? Well, it is my fault. At one point (offseason S30), I decided that the drop for ticket holders for demotees is too harsh. It was set at 30% and some teams that struggled during the whole season had horrendous cuts during demotion, giving them a really low number to start the lower division with. So, a change was made knowingly that some teams, like yours (and mine, as it turns out), will have too gentle a cut. Something most of us can live with
