Hi guys,
I'm a former Mark VI railie, and I stay in touch with a fair number of folks in the department. Here's what I've heard. I'm going to stress that it's what I've heard, and I can't represent this as absolute fact. Take that for what it's worth.
The short, short version is that Monorail Central cleared Monorail Pink in reverse into the Concourse (the Epcot side of the TTC) station without being aware that Monorail Purple was still in the station. Pink was cleared with MAPO override, meaning that the driver would have disabled the anti-collision system (there are a variety of legitimate reasons for doing this), and would have been acting on faith that Central wouldn't have cleared him into a dangerous situation because it sometimes is difficult to see everything you'd want to in the cab mirrors, particularly at night on a curve such as the one he was backing along. For whatever reason, Purple just sat in the station without attempting to avoid the accident, and was struck by Pink moving at the full 15 mph allowed during MAPO override. From what I understand, the driver of Purple was a new transfer from another department. Why Central would not have made sure the station was clear before moving any trains is a bit of a mystery, but I've heard that no one was actually in the control console (the control tower-looking place at Concourse) at the time. To my knowledge, there were no electrical or mechanical problems in play here. Again, I have to stress that I'm relating what I've heard, and I was not there. This is a second-hand account from others more familiar with the situation.