This is a long one.
When a nether portal in the overworld receives a block tick there is a small chance (ranging from .05% to .15% depending on difficulty) that a zombified piglin will spawn next to the portal. This mechanic is often used in overworld gold/xp farms by repeatedly destroying and relighting a portal.
[BUG] While this event takes place, any portal in a loaded chunk of the overworld will have mass amounts of zombified piglins spawn next to its nether counterpart. If you had provoked the piglin that spawned from the original portal in the overworld, then the hoards that spawned in the nether will be provoked as well. This results in an insta-kill upon entering the nether through an effected portal (There is absolutely no way to survive it. My full protection 4 netherite armour plus a totem lasted a whopping 2 seconds lol).
Definitely dangerous and definitely a risk of losing valuable items. The only way to currently combat this is to set your game to peaceful while you enter the portal.