How does the rotor come off without the brake caliper also coming off?
It’s hard to know exactly what happened without seeing the condition of the truck.
That said, I have been watching a YouTube channel called Just Rolled In, each episode of which is compilations of short video clips documenting the crazy and dangerous stuff that a mechanic sees in customers’ cars. The majority of these involve poor work by DIY owners, incompetent mechanics, owners’ neglect of obvious problems, and damage from road hazards. Watch a few of these videos, and you can start to understand how the problem like that truck losing a wheel could have happened.
I’m thinking the bolts that hold the hub spindle came loose or broke. For example: on a 2wd 95-04 S10/Blazer there are only 3 bolts for the spindle/hub assembly, and they are no bigger than the 5 studs that hold the wheel on. But with standard 15” rims the clearance between the caliper and rim is possibly small enough to keep the rotor from escaping the caliper.
However, the big aftermarket rims have ample room. In that case the caliper can’t hold it as the ample room will allow the spindle/hub/rotor to go with the wheel.
And the big offset rims can put the rotor almost out, or even beyond the rim, so escaping the caliper in a spindle failure is likely.
The last description is my guess in this case.