My first thought was that there was a giant mushroom cloud on the horizon behind that ridgeline that Derpy and Twilight were taking shelter from, and Derpy was apologizing to Twilight for causing it. "Oops, my bad..."
The characters do look *slightly* out of place, they have noticeably sharper lines and more detail than the background. But it doesn't jump out at me very strongly. Maybe adding a few more fine lines to the cliff face behind them would make them blend in better? The color schemes are a good match already.