That's because at roundstart you set the bool true, but you don't use it until a player changes team.
The code only sets the player to CS_TEAM_T if there is more then 1 terrorist at the moment a player switches team.
What you need to do is make a new function that holds the code to change a player's team.
And at roundstart and the TeamInfo event check if there is more then 1 T, if so call the function.