Here is the relevant portion of my (now out of date) Phoenix VFR Sectional Chart.The blue routes indicate the VFR corridors for aircraft transitioning the PHX control zone.I've added in red,the likely flight path of the accident aircraft,but the NTSB report may show different.Without a clearance for a higher altitude than 4,500-5,500' ASL,the pilot should have joined the blue corridor with a turn to the SE,thus avoiding the higher elevations of the range.

As to what went wrong,it will be published in the final report from the NTSB.
Both pilots were experienced mountain pilots,although only one was in the cockpit at the time of the crash, and their company was involved in firefighting surveillance....see the photo section...
http://ponderosaaviation.net.
Regards:SH.
May the victims and families find peace.