The thin shot is almost always a mistake that stems from an issue with your posture.
In my experience, it happens when your pelvis moves towards the ball on the downswing – often it starts at the end of a long range session when you start to get tired and all too often it quickly gets grooved into your game.
This move leads to your shoulders rising which lifts the arc of the swing and the clubface catches the ball on the equator.
There are plenty of other causes for golfers hitting thin shots. One of the main ones is “the chicken wing”.