These systems typically have a photocell, or other sensor to ensure that the gate will not come down if there's something still in the gateway. They have to be this way because cars do stop under the gate, for example if there's cross traffic or a pedestrian to wait for.

So, not knowing the details of how this happened, it's possible that the size, shape and speed of a bicycle makes it possible to enter late and not be detected while under the gate arm. Or maybe the system is defective and simply malfunctioned.

Fortunately, I'm not going to be assigned to determine liability, so other than some idle speculation, I can happily put it out of my mind.