She tried really hard to make the ball, using low left english. By concentrating on making the ball she stared at the contact point on the object ball. When you try really hard you tend to stroke through very well. This does not allow the cueball to curve, so you brick it thick. The same as happened to John Schmidt against Harriman late in their first all around match.
With low left english her aim point for her tip needs to be far off the left edge of the object ball. If she had actually been trying to draw the ball hitting it harder she's not likely to make that mistake. Hitting it easier you tend to think it won't deflect so much, but with a good stroke it will still deflect and it won't curve back so it hits thick.
It was an aiming error. In my experience outside english shots are nearly always hit thick in these situations. If you hit it too thin, it could be a bad stroke pulling up and letting the cueball curve, or just too wide of an aim. Too thick and it is always an aim error, imo.