Race condition on x86

Could someone explain this statement:

shared variables
x = 0, y = 0

Core 1       Core 2
x = 1;       y = 1;
r1 = y;      r2 = x;

How is it possible to have r1 == 0 and r2 == 0 on x86 processors?

Source "The Language of Concurrency" by Bartosz Milewski.