But lets look at what the these things do. During descent they provide control authority over roll, pitch, and yaw of the lower stage. Even though the center of gravity is near the bottom the rocket, the system is trying to keep it vertical. It is travelling down range and needs to go from moving sideways, to coming straight down. Earlier versions of Grasshopper used hypergolic thrusters on the sides but they were scrapped in favor of the fins system. So once you run out of hydraulic fluid, you're limited to the ability to gimbal the Merlin engines for controlling pitch and yaw.

If you watch the video you can see that the fins are used all the way to the point where it lands.

So my guess is they knew they were going to run out of hydraulic fuel and brought the rocket in "hot" (at a higher than planned approach speed) and didn't manage to get close enough before the fins stopped working.

If that really was the only thing that didn't work correctly, then we'll know on the 29th.