No matter how good you may be at your game of choice, as soon as you turn on that camera and microphone combo you will screw up at some point or another. No exceptions.
The Let's Players' Curse is different to regular anxiety because even after one gets over the initial nervousness that might present itself when talking to a camera before uploading your video to a potential audience of thousands, the curse will still affect them. Some attribute it to the need to concentrate on both the game and talking in coherent sentences at once, but the curse is still known to strike when the player has stopped talking altogether. It could be the unconcious fear in the back of the player's mind of messing up and failing in front of their viewers, becoming the laughing stock of many and labelled "bad at games". Or it could just be a curse. Nobody's too sure, really.