Here is the full investigation report
Aviation Investigation Report A15P0217 - Transportation Safety Board of Canada
The first 40 pages is good reading, then it becomes a bit repetitive: Unfortunately, the final analysis chapter is a mere repetition of the facts established upfront. No real judgement passed.
How could the incident happen? Two ATP's, two engines, autopilot, IFR ratings, thousands of hours each. The biggest single problem the report highlights is the low crew gradient: Professional curtesy prevents scrutiny. Or rather, each pilot may have thought the other pilot should well be able to do the job on his own. Right they were. What one would expect a PPL holder to get right: speed, sink rate, attitude.
But if it can happen do the best, if might happen to any of us. As such it serves as a good learning.