For what reasons might a taxiway be mistaken for a runway?
Essentially, some taxiways look very like runways and they might even be more visible to pilots in certain conditions. That leads the pilots to focus on the taxiway rather than the runway.

In 2007 the FAA commissioned a study into ways to avoid pilots landing on taxiways, with special emphasis on KSEA (Seattle-Tacoma). One of their goals was to address exactly the scenario in the article you linked to:


  Find a solution for the Seattle-Tacoma International Airport (SEA-TAC)
  problem by  identifying possible visual aid enhancements to prevent
  pilots from mistakenly  identifying taxiway Tango as the adjacent
  runway 16R as a landing surface.


The study mentions several reasons why pilots mistake taxiways for runways:


A pilot will generally associate an air traffic control (ATC) instruction such as “cleared to land runway 16R,” with their visual presentation of the runway configuration. In other words, pilots see a taxiway where they expect to see a runway: this was a particular problem at KSEA because until the third runway was built, it had 16L and 16R, with taxiway T located to the right of 16R. Therefore a pilot landing on 16R would see three 'strips' with taxiway T as the rightmost one and might assume it was 16R. KSEA now has 3 runways with taxiway T between 16R and 16C.
Some taxiways look physically very like runways, i.e width, surface material, square edges etc.
Some taxiways stand out more clearly in low light conditions because their surface is lighter than the runways
Sun glare can hide runway and taxiway markings so that pilots can't tell the difference visually


There's also an FAA engineering brief that explains how to identify potential problem taxiways, it gives essentially the same information as the study.

Both documents say that the most important point is to alert pilots in advance about the potential problem by giving warnings via ATIS, charts and so on. That's especially true for pilots who aren't familiar with the airport, and GA pilots make more mistakes than airline pilots so presumably training and general currency/professionalism as well as having at least 2 pilots in the cockpit makes a difference too.