Overworld mobs in large amounts spawn on the nether side of nether portal

Multiple times when using a portal from the overworld to the nether there is a lot of mobs gathered on the nether side resulting in instant death before the screen is even loaded and multiple creeper explosions. Both gates are spawn proofed in closed areas and well lit. This has happened multiple times though multiple gates. It could not have been a ghast because the portal is underground in the nether.

A few times where we survived the loading screen there were multiple skeletons, spiders, creepers etc. right on the gate (in the nether).