When a tile is dragged and dropped towards the middle of the table it is considered a discard. We sometimes see players moving tiles around in their racks in an upward arc and then accidentally dropping one in the discard area along the way. This is more likely to happen when using a mouse, especially when it's coming to the end of its battery life. But, it can also happen when using a touch screen, such as an iPad (especially if the screen is dirty). So, I would suggest making sure that you drag tiles horizontally in your rack and not in an upward arc.
Another action that will discard a tile is the double-click. So, be sure that you don't accidentally click then click again within a short period of time (sometimes this can happen as an automatic reflex, because we're so used to doing it!).
If this issue did happen while you were rearranging your tiles, to be super safe, you can choose to only re-arrange your rack when it's not your turn. This way there will be no chance of accidentally discarding.