This happened not on our X3, but on our Lexus when I accidently spun out a few weeks ago. I'm guessing maybe inadaquate engine lubrication from g-forces. That, or the fuel tank thing you mentioned. Basically, it has to do with something not reaching the engine.

I doubt a hard U turn will flip an X5 unless you do something stupid. I did a hard U turn at 40mph in the X3 once and all that happened was a bit of tire squealing and DSC flickered a couple times. Heck, I think I was more in shock about how easily the X3 took it as opposed to "holy crap I almost died". Seriously, the X3 shrugged it off just like "so what? Is that all you can throw me?"