This post may get a little too technical, so bare with.
The overall speed of the ride has absolutely nothing to do with it's mass. The speed is purely gained from the energy lost as gravitational potential, converted into kinetic energy (movement). I'd throw formulas out, but basically if you've seen a feather and a big heavy thing dropped in a vacuum, you'd know what I mean. The one the the mass does affect is the momentum of the train (momentum = mass x velocity). With less mass the change in momentum is now more dependant on the speed of the train, which only has one maximum. With a very windy day and less momentum on the train, it would be possible for it to lose much more speed than if it was a full train. Basically as a stall hadn't happened since the day of the google earth picture in 2013 (is that right?), the stall is a direct result of a lower momentum and high winds. I know the PMBO won't operate in high winds, etc etc.
Because stalling hadn't been a problem for such a long time it would sort of make sense to stop operation if the wind was too much, or make sure every car was full if the wind passed a particular amount. I guess it's a bit like th13teen can't run empty and the trims up.
Maybe this wasn't the most constructive post, but that's a sort of more technical point of view as to why it will have stalled, particularly if all of the trims were down from a slower speed.