The talk was that a mechanical failure resulted in at least one slide not deploying. I've heard a few scenarios, but the overwhelming ones are either a canister did not release to inflate a slide, or that a door was off by fractions of an inch and did not open properly. - or both.
Regardless, all has been carefully scrutinized and we should be hearing something early next week about the re-take of the mini-evac.
Fingers crossed: we get this one right, and Proving Flights could begin before Christmas.