A mob's death on top of a Sculk Sensor causes the Catalyst to spawn a Sculk Vein on top of the Sculk Sensor. This seems to only occur when the mob is on top of the Sculk Sensor or within a close proximity to the Sensor when it dies.

Steps to reproduce
1)Place a Sculk Catalyst and within its proximity to spawn Sculk, a Sculk Sensor.
2)Kill a mob in such a way that when its death animation plays ( the mob falls to its side and drops items) it lands on top of or within 2 blocks of the Sculk Sensor.

I have tested this with other mobs affected by gravity such as a cow, ocelot, chicken and wolf and the same outcome occurred as with the pig. Not sure if this qualifies as simply an extension of Beta Minecraft being experimental and some features not being fully polished yet or as a legitimate bug, but felt it best to record it just in case it is a legitimate bug.