It's not a hack, but it's still a breach.

A data breach just means that data is accessed by unauthorized parties. Accidental data breaches are not only possible, but common. If a company accidentally emails a group of customers with CC instead of BCC that's technically a data breach, although in most circumstances a low-impact one.

Breach used in this case is really spin. Calling it what it is, a "inadvertent disclosure" puts the blame where it belongs.

We call it a breach because the original sense of the word refers to an opening, tear or rupture. In that sense, this is exactly the correct term. Who or what caused the opening is irrelevant.

Hey man I'm just trying to get done work, breach my front door and than breach a few cold ones.

If it's actionable by the ICO, it isn't a low impact one either.

If a bank left $1b of cash at a bus stop and somebody takes it, it's not an explicit bank robbery, but it's still a theft.

This however is worse, as the property they were negligent with wasn't their own property. It's the bank taking your safe deposit contents and leaving it at the bus stop.

Ok, we've made the title say leak instead. Thanks!