Dissonant Whispers states:
You whisper a discordant melody that only one creature of your choice within range can hear, wracking it with terrible pain. The target must make a Wisdom saving throw. On a failed save, it takes 3d6 psychic damage and must immediately use its reaction, if available, to move as far as its speed allows away from you. The creature doesn't move into obviously dangerous ground, such as a fire or a pit. On a successful save, the target takes half as much damage and doesn't have to move away. A deafened creature automatically succeeds on the save.
I had an incident where a PC was being grappled and bashed about by an Otyugh. The character was "being flailed about" in the grapple (which also restrains the PC), and used Disonnant Whispers successfully. I had the Otyugh release the grapple by chucking the PC away from it as it's reaction (like an tentacle slam, but away and 2d6 falling w/o stun). It moved the frightening thing (PC) away from the Otyugh, which would have otherwise been dragged in the grapple, and since it used it's reaction, it stayed where it was.
The players, of course, were like "Booo" but I honestly think it would have fit the scenario, since the character was being grappled/restrained by the tentacle and was already being flailed about. Did I make an unfair ruling?
Best Answer
I have a question for you. If you are walking around and suddenly feel a tickle on your hand and look down to notice a spider that might be a black widow crawling on your hand, do you take the time to wind up and try to fling that sucker for distance, or do you panic and flail wildly trying to throw it off before it can bite you? The spell instills fear, so ignore for a moment that you are a cool and collected veteran of many battles and have absolutely no fear of a simple spider, assume you are deathly afraid of the little beast.
Likewise, the Otyugh might give you a slight push as it releases you, but would mostly just be dropping you so that it can put as much distance between you and it as possible, as quickly as possible. It is quite loathe to touch you for a mere millisecond longer than it needs to, and so would not be able to fling you with much force. Mechanically, the most it could do prior to fleeing would be to let go of you (releasing a grapple takes no action), and by the intent of the spell, it would be obligated to do so (in order to move away from you).
So, the interpretation in the OP was turning an intended negative effect of a spell into a benefit for the victim, which IMHO is a poor ruling.