It was due to an incorrect manual MAPO override permission, given by controllers in the central control room. This enabled the driver to pass a red signal - moving in to a section that the controllers believed was clear, but actually housed a stationary monorail in the station. Thus crashing.
Monorail system was pretty much back up and running straight away. Just with 10 out of 12 monorails running...
As a mark of respect for the drivers, front-seat riding (in the driver's cabin) has been suspended.