Right, I'm confused by this. Surely the pilot would notice at least one of the planes he'd be inevitably crashing into well before he started to land?
In theory yes, but the risk is that the plane you crash into is not yet on the taxiway or that poor visibility or tunneled focus prevents you from noticing it.
You don't need to be moving quickly to have a significant accident. The deadliest aviation accident in history (583 fatalities) resulted when two planes collided on a taxiway (edit: actually on the runway) in dense fog [1].
Edit: as correctly pointed out below, the actual collision took place on the runway at significant speed.
LOTS of planes accidentally line up on the taxiway if visibility is reduced and cockpit workload is high. Heck, aircraft have even taken off on non active runways at the most advanced airport in the world with disastrous results at night. [0]
Inclement weather on approach at the end of a long flight can also lead to crew pressing home while under confusion, as in the case of the Korean airlines accident that landed short. [1]
More recently, at SFO too, the Asiana B777 crash was something that should have been almost impossible in a modern jetlines, but over reliance on automation and lazy piloting lead to that disaster.
Which seems to be what happened here - the pilot aligned with the taxiway, noticed he was in the wrong spot, and the tower told him to try again.