Had a look at the pic (having turned it the right way round first!) but I can't tell that its lost lots of leaves, looks quite leafy, how many leaves have fallen and were they from the bottom or the top? Any other problems, like have you checked the stems, branches and backs of leaves for signs of infestation or disease?

So far only the branch nearest the camera seems to be affected. Did the leaves look unhealthy in any way before they dropped off?

I'd be more concerned about the distance of the tree from the fence. This is a big wide tree at maturity and will overhang your neighbour's garden by several metres, if it doesn't damage the fence first. Sorry to mention this when it wasn't relevant to your question but considering this now could save you a lot of time and expense later.