Possibly misses and parries. When an attack doesn't land but the runes are not consumed, this can lead to GCD capping. If there are a sufficient number of misses and parries, the 1 second GCD of Unholy Presence will be a DPS increase.
That said, I don't have any mathematical way of evaluating whether this holds true for you personally and it might just be randomness in your specific case. Likewise, I'm not able to say if using unholy presence for a specific fight with miss-and-parry mechanics will be a good strategy. Instead, I just want to point out the general principle.