Have you carefully inspected the remaining wheels? I would check the outside and inside of all wheels as well as the lug holes. There are crack detection spray kits available for about $100.
Theoretically a steel item that is not over-stressed has an infinite life. Not so with aluminum.
But a small defect in a steel item can result in a stress riser with localized over-stress, resulting in a crack propagating out of the area. And once you have a crack, all the bets are off.
Don't think weather related temperatures would have anything to do with this failure. Heat treating that wheel (eg getting it cherry red) might even have been beneficial. But once you have a crack - all the bets are off.
I still think this is a fatigue crack. I would check the other wheels - mangniflux or some other crack detecting system.
Oh, and wheel failures due to the use of radial tires on wheels designed for bias ply tires? What I remember was rim flange cracks and increasing the gauge of the steel solved that.
I’ve had multiple trailer wheels break like that. They were marginally rated for the loads we were carrying. Upgrading to a bigger/heavier wheel/tire fixed the issue.