A CS:GO player with good gamesense will habitually keep their crosshairs at head height and aim at corners where an enemy is likely to emerge. They'll have an intuitive sense of how long it takes to run from one point on the map to another. They'll listen through walls for footsteps to try and decode where the enemy are, where they're headed to and what strategy they might be about to attempt.

To the uninitiated, it looks a lot like cheating - you peek through a window and instantly get headshotted before you've had any chance to react. To the guy who hit you, it's just basic gamesense - you did a predictable thing and he punished you for it.