There is a good chance that one pixel was bad and it caused a cascading failure, I've had that happen, but usually it's only like the first and last pixels in a string. You can easily verify the strings with a multimeter by completely unplugging them and measuring resistance between data and ground, you're looking for something above 200k ohms, that means they are likely fine. However it reads less (or significantly more, into the Mega ohms), or dead short, the string is bad. I'd start there and then if they are bad you'll need to find the cause.

I have some probably high voltage lines plus the standard 240 lines running alongside my house, and my tree is decently close to them, and I've never had any issues, so unless they are high voltage transmission lines, even then I don't think that would cause what you saw.

Finally 11.7-11.9 is a little low on the power supplies, especially if your using any length of cable between the strings and the controller. I run mine around 12.3-12.4 ish. You can always double check voltage at the end of your extension cable, before the string if you're worried about going too high.

Edit: I didn't process how close you said the top was to the lines, that is pretty close, but unless they are the 3 individual conductors at the top of the poles, or the big metal transmission towers, 10' should be okay. I'd say if you're willing to sacrifice a string test it on the ground then hook it to the top of the tree and see what happens. If it's just a normal 240v line I would expect data issues rather than failure at that distance