But you see, she has basketballs coming out of her ass.
Things coming out of an inferior orifice is easily related to laying eggs, even if they came out of her anus instead of vagina. Hence the eggs part.
Now, pay attention for this next part. Niggers play basketball, they like it, they're good at it. Niggers are very related to basketball; thus, a joke is made in which we call small basketballs "nigger eggs".
This also make the assumption than the girl was fucked by a nigger, and it's made fun of. Also, eggs cannot be laid by mammals, so it's even funnier because you call a nigger something that's not even a mammal, perhaps a reptile.
Do you see, now?