I've read various reports that seem to explain things, but leave out a few details that I'm wondering about.
1) The train with the passengers, sat on the first(?) lift hill for 5-15 minutes. Was it in fact the first lift hill, or the second or a brake run?
2) While it was sitting, this means the empty train would have been stopped somewhere as well. Was it on the second lift hill or a brake run, or had it already valleyed and sitting at the bottom of the batwing?
3) Ultimately, the question for me is where was the empty train when the first was released from its hold position?
If it had already valleyed, then clearly, maintenance screwed up by not doing a visual check and then releasing the loaded train. But, if it was stopped at another section, then valleyed after being released, then that would point to a block not being registered and/or a brake section that failed when it should have stopped the loaded train.
4) One more question. I have a basic understanding of how blocks work on coasters, and sensors indicate when a train rolls past one registering that it has passed that section. But, what happens when a valley occurs and it rocks back and forth over the same sensor? Could the sensor/computer have registered the empty train as being past that block, but not been able to detect the rollback?