One reason I've noticed is 'pot hunters'.
A team works their way up the divisions until they stop winning. After a while at this level they disband then reform under a new name and start in a much lower division where of course they cream all opposition. I know a couple of people who have left teams for just this reason as there is very little development past a certain point as they never stick around long enough playing in the higher divisions.
This also has a detrimental effect on new teams in lower divsions who find themselves up against seasoned players who are just out for more trophys.
A team works their way up the divisions until they stop winning. After a while at this level they disband then reform under a new name and start in a much lower division where of course they cream all opposition. I know a couple of people who have left teams for just this reason as there is very little development past a certain point as they never stick around long enough playing in the higher divisions.
This also has a detrimental effect on new teams in lower divsions who find themselves up against seasoned players who are just out for more trophys.