Interesting theory from the article's comments section (Obviously speculation but seemingly plausible):

"This is a horrific tragedy. At 08:48 AM the bright sun on this cloudless September 18th morning would have been at 20.4 degrees in elevation from the horizon, at 109.4 degrees from True North or about East South East. Sunlight would have passed through the driver's windshield and been reflected off many bright surfaces within the bus and reflected off the inside of the driver's windshield into the driver's face just before the corner. As the corner was rounded, the driver would have been distracted by the windshield glare and internal reflections of passengers taking their seats, just long enough to experience a phenomenon called saccadic masking. Unfortunately this could happen to any bus driver under these conditions. Saccadic masking happens even to the most skilled and experienced fighter pilots. If you were to ask how this could happen, there are many scenarios, but saccadic masking seems to be most likely. In order for the bus to round the corner, significant physical input would be required by the bus driver, so likely the driver was conscious at impact.
Unfortunately, this could easily happen again. Buses west bound on Carling at sunset are at risk."