How to reproduce:
1- Create a small rail "round" with a minecart
2- Spawn a Spider
3- Push the minecart toward the spider so the spider rides it
4- Hit the spider with a sword, the center of it's body, which is inside the minecart. (Not the parts which stretch out through the cart.)
The spider sink into the solid ground bellow and start to suffocate.

Work better in creative because the spider stay into the ground instead of trying to jump at you.