Relegated after finishing 11th last season?

I was relegated after finishing 11th in last league season.

Is there a particular reason for this or is it just a roll of the dice?

This is standard procedure. Numbers 1-4 gets promoted (unless already in 1st league, of course), numbers 5-10 stays in the same league, and numbers 11+ gets demoted. For other games the numbers are 1-2, 3-5 and 6+. There are exceptions, though. If you have a high enough rating, you may get to a higher league independently of your placement in the current championship.

ok. I've seen many occasions where 11th position have been enough that I thought it's the last position to survive.

I also thought 11th was the last safe position regardless of whether there were 18 or 25 in the StreetSoccer group.

I think 11th is supposed to be safe. With a normal size section in other games (9) the the bottom four go down, which is twice the number who go up. In soccer, I think the normal number in a section is 19, with 8 going down (again, twice the number going up). That would mean 12-19 get relegated and 11 stays put.

According to RoRoRo, 11th place is relegated.

I thought the "normal" number in a soccer tournament was 18. However, there are almost always more people in a spesific league, due to the "special rules". I could be wrong, of course.

Championship #23 seems to have smaller groups than usual. Only 14 in the top division!