Here's a pic with the rotor out of the way...
View attachment 174493
Just a theory, but since this happened on a left side wheel, with right hand threaded stubs and the forward movement of the vehicle, it would take a long time for the wheel nuts to come off. In fact, I've experienced a situation where months have gone by with lug nuts loose and they did not come off.
I suspect you had several lug nuts missing for some time and didn't realize it, and I've seen that, too. It could be a possibility that someone removed some, either because they didn't want to buy them, or someone has a grudge or playing a (dirty) trick on you.
The broken studs are likely the result of the severely loose wheel turning off centric putting too much lateral force on the stud, which got progressively worse has the hub lost studs.
The "holes" in the back of the wheel are casting vents. Not noticeable in the photograph, but there are small slots in those cavities to allow air in.
Sorry to hear of your mishap. Glad nobody was injured.
Best regards,
Dusty
2019 Ram 1500 Silver Billet Laramie, Quad Cab, 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, 18 inch wheels. Build date: 17 April 2018. Now at 018904 miles.