Usually, it's because the sperm gets into the fallopian tube before the egg is released. Once there, the sperm wait for an egg to arrive. After fertilization, it can take 5-10 days for the fertilized egg to begin to attach itself to the lining of the uterus. Pregnancy begins when this happens.
The arrangement of spikes originally had no distinct name; the term Thagomizer was coined in 1982 by cartoonist Gary Larson in his comic The Far Side, and thereafter became gradually adopted as an informal term within scientific circles, research, and education.
The term thagomizer comes from a 1982 Far Side strip featuring a caveman lecturing to a class about dinosaur anatomy, the joke being that the anatomical feature was named for a fellow caveman who presumably died as a result of contact with it. (Fun honor, becoming the namesake for the thing that killed you.