Two things could be happening:
Dragging in an arc: When you drag your tiles, you might be moving then in a arc and accidentally dropping a tile into the discard zone (the table area above your rack). Try dragging your tiles straight across, horizontally within your rack.
Accidental double-clicks: Sometimes players double-click a tile when they meant to single-click. It’s more common than you’d think!
We have 2 options can will help avoid accidental discards (both options are located under the "gear" menu, just above your tiles, to the right ):
1. Discard actions: you can choose to disable double-clicks or drags as a discard action (depending on which of the two scenarios above you think is happening)
2. Discard confirmations: this will ask to confirm each discard, so if you're about to accidentally discard a tile, you can cancel out of it
If you’d like to be extra safe, consider rearranging your rack only when it’s not your turn - that way, you can’t accidentally discard a tile.