See
http://avherald.com/h?article=4ba33b1c for a nice writeup and METAR.
It's mostly speculation what exactly happened until a report is released, but by all means lets speculate:
The wording "last minute deviation" may indicate they were doing an autoland and the localizer was disturbed by the departing plane or something else.
Maybe they decided to go-around shortly before touchdown but of course you need to put in a bit of energy and they still touched down and that was with the left main gear into the grass area to the left side. (because they were not lined up correctly)
Or:
They could have bounced on landing, decided to go-around and then during the go-around deviated to the left.
That still fits with the location of the the runway excursion right at the end of the touchdown zone.
10C where they would normally land was closed at the time but i'm not sure why. (was a planned closure didn't find any go-arounds due to sudden closure)
Maybe a runway inspection or too much rain?