Have you ever kicked a PC from a party?

[Frozen_Feet]

More often they just leave, but it's still a common occurrence. My players don't have to play their characters as part of the same group, so they split fairly often. It's a player's choice if they want to keep playing a separated character. If they don't, the character becomes an NPC and fades to the background. The player can roll a new character if they want to keep playing, otherwise they're out of the game. If they do want to keep playing their old character, the game shifts to follow that character during their turn until said character reunites with the rest or dies. (Sometimes they get lucky and all the others die instead.)

Usual causes:

1) characters don't along. F.ex. hippie halfling gets upset about killing monsters and leaves the group.

2) characters really don't get along. F. ex. one guy tried to murder another in their sleep, the other guy retaliated, leading to death of the first one.

3) accident happens. F. ex. a cave in separates characters from each other.

4) it's planned. F. ex. half of the party go forward while the other half go to fetch backup or look after the wounded.