I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
My best evaluation is that something is stressing the tree's roots. Dry soil or soggy wet soil both are damaging to the roots. Evergreen trees such as the eucalyptus do drop leaves periodically. If only the older or "inner" leaves are affected while the tree is still producing normal new growth this may be normal and I would not worry about it. You might also consider if some weed control product was applied that may have washed down into the tree's root system. Time is the only solution to weed killer injury.
Thanks for the question. Please stop in again soon!
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