I don't know, but you are more likely to get a helpful answer, the more information you can give. To start with where are you located? There are hardly any garden questions that can be discussed without regard to location/ climate. Then, more about what has happened -- when did you first notice the problem, how long has it been going on? You said you had trees in that spot for 8 years, but how long had this particular tree been there? Any other symptoms? Did the leaves just all fall off at once or a little at a time? Did they turn brown first or curl up or get spots or .... ? Any discolorations or lesions on the trunk or branches? Try snapping some twigs off. Are they still green inside? If not, try scraping the top layer of bark off a very small area of trunk. Is there green under there? If no, then the tree probably is dead and past reviving.
And please post a couple pictures of your tree now and if you have any pictures of before the problem, during the problem.
Since the other two are OK, I expect it is some disease or pest.
Here's a couple articles about diseases and pests of citrus trees, so you can see if anything looks familiar:
http://www.ipm.ucdavis.edu/PMG/C107/m10 ... trees.html
http://www.gardeningknowhow.com/edible/ ... -trees.htm
Hope we can help!