In other words, what caused SpaceX to miscalculate the amount of propellant required to safely land the booster. They had landed successfully on the drone ship in April. So what went wrong here? What was different?
When performing a propulsive landing, the harder you accelerate, the less speed you pick up from gravity, and the less fuel you need.
Falcon 9 flight 26 was intended to perform a three-engine landing burn. According to an Elon Musk tweet, one of the engines wasn't operating at the intended power level. This reduced the acceleration and increased the fuel needed: although the booster had enough fuel to perform a three-engine landing, it didn't have enough for a two-and-a-half-engine landing.