A manager looking after a clients money may be paid a commission that is based on the amount traded. This creates a temptation to churn. This means trading more than is beneficial to the client.
Churning may involve things other than securities trading. It may involve insurance policies, pensions, and almost any kind of investment or financial agreement that is sold on commission.Discretionary stock broking services are usually paid for with a fee based on the value of the portfolio, rather than through trading commission. This eliminates the motivation to churn. This does still leave a temptation to encourage advisory clients to trade more than necessary.
Churning clearly breaches a duty to clients, and is usually a breach of regulators' rules. However, unless the level of trading is so high as to be clearly excessive, churning can be hard to prove.
Ultimately the fundamental cause of churning is an agency issue. Paying sales people commission creates a conflict of interest if customers rely on their advice.