What are some causes for long chains?
There's an extensive discussion on long chains here.

A "long chain", as defined by bc.i, appears to be a transaction chain whose growth rate exceeds a target value in 24-hour period. Note that transaction chains are destined to become long over time. That's how chain of ownership works. It seems the thing bc.i wants to separate out is rapid velocity transactions.

A long chain requires that a coin is given to a payee, who quickly passes it on, and so on. It's a sign of either highly coordinated effort between multiple users (unlikely) or a single user shaving a coin by making rapid-succession payments.

There are many possible causes beyond the ones you cite. For example, the game SauruTobi supposedly used high-velocity transactions, and from what I understand, so does ChangeTip. Essentially, any direct-payout service can cause long change, which covers a lot of ground.