Well, any time you're uncomfortable, you're prone to an ugly swing. (This could explain why I hack my way around a course - I'm never comfortable!
)
Here are the usual suspects for popping a chubby:
1) Sway. You slid back on your backswing, and stayed there for the follow through.
2) Casting. The clubhead beat your hands to the ball. By a landslide. And now, land has slid. Because your hands weren't leading, the clubhead slipped forward and bottomed out before ever reaching the ball.
3) A shoulder dip. This feels like a good move towards the ball, but instead tilts your spine away from target, and drops your effective spine angle. The result? Loss of power (because you didn't really uncoil, now did you?), and now your club makes a better shovel than a ball-mover.