1) Two exclusions by other DT1 members.
2) Theymos forcibly him by removing DeadTerra from DT1. I think that this option is the better one.
I also think 2) is a very good option to go with since DeaDTerra is inactive since 2015. Excluding Cecil is not the only option to deal it. There might be many like in DeaDTerra's list.
CecilNiosaki uses [email protected] here http://archive.is/P7XDM
DigitalGangster forum db was hacked in 2016 as according to vigilante.pw. Encrypted password and salt of password of the DigitalGangster forum account is publicly available. Possible that it might be hacked if he used the same password over both forum