(Disclaimer: I'm a fairly novice Ardupilot-er, but any free opinion is better than no opinion!)
I notice what looks like 4 separate lost-link events in the C1 and C2 plot. (3 of them recover, of course)
I've never used Turnigy, but the Futaba RX that I fly have programmable failsafe conditions. In a lost-link case, the receiver commands the pre-set values for all channels. So if your failsafe condition has C1 at 1000 (standard "min" value) and C2 at 2000 (standard "max" value) then lost-link events explain the plot, and possibly the crash. If you haven't modified your TX and/or RX settings post-crash, you can test this hypothesis.
As for what caused the lost-link... I might speculate just for fun.
Idea #1: can you view the CURR and/or POWR in the log? Maybe find evidence of a brownout or blackout? ESC# logs might also show if there was some internal power issue?
(crazy) Idea #2: You said you impacted a power line? Would a high-current power line be able to interfere with nearby weak radio signals? That might explain the lost-links, but I would expect the radio link to recover as the quad fell away from the lines. (So I don't particularly like this idea.)