Lately I've been seeing a lot of executes going off on champions with nearly full health.
I see some questionable predator/voli executes happen towards the end of a fight. It's particularly noticeable with champs that have a lot of healing, the threshold seems more like a % of max health damage taken over the fight rather than a current % health.
Most recently I had a level 2 maokai 1v1ing a level 2 volibear, and he shrugged off all of the damage voli was doing for the whole fight through armor and mr stats. Each bite voli was doing only did about 1/4 of his health, which he healed back. URF kicks in, and suddenly Voli's next bite kills maokai while he had about 90% health.