What a tragedy. My thoughts and prayers go out to the family of the deceased Monorail pilot as well as the pilot of the other Monorail. Imagine how they must feel.
On another Disney discussion board, there was "unofficial" information regarding the crash - but from a reliable source (former Monorail Pilot with contacts at WDW monorail). According to the post, the Monorail Pink was given clearance from Monorail Central to reverse into the Concourse without being aware that Monorail Purple was still in the station. Pink was cleared with MAPO override, which means the driver would have disabled the anti-collision system. Monorail Purple did not attempt to move away from Monorail Pink - which was traveling at the full 15 mph allowed during MAPO override. The poster also stated they were told the Monorail Purple Pilot was a new transfer from another department.
Whether or not turns out to be true it is a plausible explanation for what happened based upon the photos and video that have been shown. Regardless, it's a sad situation no matter what the cause ultimately is determined to be.