Even with the chain tight, it can still come off due to poor alignment. To check this, look at the top of the rear sprocket and note where the sprocket it riding in the rollers of the chain. You will need to adjust the tensioner on the brake side to keep the sprocket perfectly straight in the chain. You can also look down the top of the chain from the rear sprocket to the front sprocket and tell if it is REALLY out of alignment.
If you have crashed hard on the sprocket side, the sprocket bends easily and you may need a new one, the same goes if the sprocket has ever come loose and popped the chain off. I would recommend taking the sprocket off, getting 5mm longer bolts and putting blue loctite on them before reassembling.