I'd be guessing to offer an opinion, but I will.  First, and most important, I'll wager every one of these vehicles has an automatic transmission and I'll further wager every one of them were in Drive.  They should be in Low or 1st gear.  They all have the lower gear, but most drivers here don't know what it is and don't use them.  The transmission can be a terrific brake without locking up wheels.  Second, I blame the city because they should know these foothill roads should have already been plowed and keep plowing.  No evidence of that early morning.  They're probably down the hill on the flat making sure main street and such are plowed.  Dumb.  Third, I fault the drivers again.  Some of thise vehicles appear to have a 4WD option and they probably all have it engaged thinking they will have traction on a downslope.  They ignore the effect snow and ice and 4WD have on braking: there is no effect.