Given its stalled in that same spot before, there should be CCTV pointed at it that operators can see, tbh. I dunno whether there is or not, but it seems like common sense?
My theory is..... Minor breakdown
, engineers come to reset, send all 4 trains for a test, all seems well but train 4 has stalled due to light weight and wind, they load train 1 unaware and dispatch it, when train 1 hits the lift it shuts down because the block ahead isn't clear (stalled train), engineers are still around and assume the ride is crying wolf again and reset without looking properly. Maybe whatever error that shows up when a block section isn't clear had just occurred, causing the prior breakdown, and it was a freak coincidence that it then actually happened.
This is just a wild guess bases on what little knowledge I have.
What this accident shows is that h&s procedures, even if they seem dumb, are important. You can imagine the frustration of dealing with an error keep coming up that is never true and then this one time it is, not paying full attention to it.
When I worked at a park, guests occasionally claim their restraints fail. Of course they're mistaken, all the time, but you take it seriously just on case. Just. In. Case.
I was thinking today - has there ever been an accident of two trains colliding after a stall, ever? And more importantly, at a major park on a modern, major ride? This really is a rare and weird accident.