Uh, yeah, they screwed up. That simple.
Normally, when landing in SFO on the 28s, which is most of the time, planes follow a published approach. That could be an ILS, or an RNAV, or a charted visual approach. Maybe Air Canada followed one of those approaches until going visual and then lined up on the taxiway. At night, planes that are on the ground are hard to see, so Air Canada might not have seen them, but runway lights are easy to see. SFO 28R and 28L are lit up like Christmas at Clark Griswald's house. And there are approach lights with big, flashing strobes to get your attention. On the other hand, taxiways are barely lighted, but at least taxiways use different color lights.
So, those guys screwed up. Maybe they were tired. Maybe they were both colorblind. Maybe they were both playing Angry Birds on their ipads. It doesn't matter, though, because they screwed up.
A similar event happened in BOS a few years ago with a general aviation plane. The plane almost landed on a taxiway, and one of my friends who was working, but a tower supervisor told the plane to go around. Close call, but no crash.