If you go to Ellsbury's Hit Tracker page, you'll see that he had far fewer than the normal percentage of "Just Enough" homers in 2011--just four out of 31 (one of the 32 was inside-the-park). So he wasn't lucky. Either we think he was juicing, or we saw a peak expression of his real abilities.
Now, it's not only possible, but highly likely, that 2011 was Ellsbury's career year. But "career year" is one thing; "fluke" or "outlier" is another. I think it's far more likely that we're seeing the aftereffects of a shoulder problem and a long layoff than that Ellsbury has simply turned back into a pumpkin. I don't think he's a .900 OPS guy going forward, but I think .850-ish is a good rational-optimist expectation through the remainder of his prime (say the next 3-4 years).
That is only one type of home run luck.
You can be lucky in choosing which pitches to swing at. A pitcher's pitches vary a lot during a start. While a hitter chooses to swing at a pitch, sometimes that fastball that just happens to have a little less rise or that curve ball with not break as much and the hitter makes better contact. That's still a form of luck. Additional luck includes wind currents, quality of pitcher, quality of pitcher that day, focus from other team's scouting departments, etc.
I don't doubt that Ellsbury has the strength and ability to hit home runs. An 850 OPS certainly isn't crazy. I think he will be worse than that, probably around 825 but I am not wed to that. He has done better so the possibility remains for him to be excellent.