You are correct in that the rotor disk is still attached. However, in regard to a spindle fracture as being the probable cause of the wheel coming off, there is another possibility with a high probability. A few manufactures no longer actually use spindles on their light duty trucks they use a bolt on hub assembly Chevy and GMC for their HD 2500&3500 models use a 4 bolt hub assembly with 14MM bolts, their 1500 models have only 3 14mm bolts, the hubs are nearly identical for 2wd or 4wd the difference is some will not be splined for the CV assembly
Installing wide tires on spacer s and those near zero sidewall tires are as you said a huge factor in failures
Here is a picture of a hub I recently replaced on an HD2500 4WD
Attachment 45614
And here is a picture of a 1500 2wd
Attachment 45615
Edit I just did a search and this type of bolt on spindle hub seems pretty much industry wide