Braking Technique

In the footage of Beloki's spectacular crash it can be seen that Armstrong
briefly locks up his rear wheel as Beloki crashes. I thought on a decent
surface the best way to stop was front brake only? I guess that was feasible
with the Weinmann brakes on my Eddy Meckx but these Tektro cantilevers leave
some braking performance to be desired . Anyway, obviously on a descent
like that you'll want to use both to distribute the rim heating evenly, but
why did Lance lock up the rear? In the emergency situation shouldn't he have
been using the front brake exclusively -- exactly for the reason of avoiding
a rear wheel skid?

(I will agree that front brake only is a little "eggs in one basket" because
if you wash out the front you're pretty much gone, but I'm arguing based on
the "front brake only" rule I've heard time and time again.)

Dani