Shopping cart caster

what is the scientific explanation of shopping cart wheels to turn in
opposite direction while travelling in some direction? what kind of
forces act to make them behave like that? is it related to weight
bias? or is the friction force changes according to the caster angle,
positive or negative?
i try to find answer to my questions from the following excerpt but it
did not seem to help so much.
thanks in advance

"The best way to visualize caster is to picture a shopping cart
caster. The pivot of this type of caster, while not at an angle,
intersects the ground ahead of the wheel contact patch. When the wheel
is behind the pivot at the point where it contacts the ground, it is
in positive caster. Picture yourself trying to push the cart and keep
the wheel ahead of the pivot. The wheel will continually try to turn
from straight ahead. That is what happens when a car has the caster
set too far negative. Like camber, on many front-wheel-drive vehicles,
caster is not adjustable. If the caster is out on these cars, it
indicates that something is worn or bent, possibly from an accident,
and must be repaired or replaced."
http://www.eastcountyalignment.com/understa.htm