Smartcast being automatically enabled! Why is this?

For some reason Smartcast is being automatically enabled when I play. I selected the 'restore to default' in the settings and it was fine for one game, however on the next game it re-enabled itself. Does anyone know why it is doing this? I haven't enabled it myself in the settings.

Example: I understand that some champions have automatic casting (Vayne's Q) but I was playing Ashe. When I cast Ashe's W I usually have to left-click immediately after, however the ability was casting as soon as I hit W (I didn't have to left click.) Why is this?