Pigmen Over Spawning On Nether Portals

Put the summary of the bug you're having here

What I expected to happen was...:
The nether portals should link up fine and no zombie-pigmen outburst would happen

What actually happened was...:
Huge lag spike caused my hundreds of entities (zombie-pigmen)

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. link up two nether portals through the nether
3. zombie-pigmen will burst through both sides in the overworld