Connecting an LVDS/eDP cable into a live motherboard

[ResoluteHawk]

Could anyone explain the reasoning behind why connecting an LVDS or eDP cable into a laptop motherboard, with the battery still connected, kills the CPU/PCH/SuperIO/GPU?

Today I had a customer with a Lenovo Legion 5 that I diagnosed as a dead iGPU inside their Ryzen 5900HX. They told me that they connected their screen to the motherboard without first disconnecting the battery.

I want to understand the nitty gritty behind why it kills components if power is not disconnected first.

[m1ch43lzm]

Usually you have the main power rail voltage (after a fuse) on some pins of the LCD connector for the backlight even when the laptop is powered off, either from battery or AC adapter, when unplugging/plugging in the cable your customer may have wiggled the cable a bit to the sides/inserted the connector at a slight angle (it's hard to get those connectors straight first try), temporarily shorting the main power rail/creating a spark, it depends on the pinout of the LCD connector on the board
On some cases you're lucky the only thing blown is the backlight fuse, just to save few minutes taking the bottom cover off and unplugging/removing the battery, when doing a screen replacement

That's why it's advised to unplug the battery first when replacing/upgrading components (screen, RAM, SSD) or even when replacing thermal paste, a screw can fall somewhere on the board and short something...