any rain or wet surface at that time ?
I suspect ABS didn't work properly ,normal brakes works if ABS fails
but a malfunction ABS can cause such a accident specially on wet or slippery road
i was divining a 2008 Dodge Van (normally a very safe and reliable family Van) in USA
the road surface was wet because of the light morning snow
in our town there is a street with a medium steep of just few yards immediately followed by a
stop sign.while going down i applied breaks and instead of stopping my van start shacking (some
Girr girr noise from brakes) and van crossed stop sign with speed .thanks it didn't hit any school kids ,people
or other cars ..i checked my ABS by mechanic later on and ABS was working properly
i can only guess "on rare occasions vehicles with ABS can malfunction or on steep roads can do this"
i am saying this because i tested my van brakes on even surface immediately after accident and
van brakes were working .
may be your uncle had a same type of accident