I planted a Blue Maid holly - about 3 feet tall - last Oct. and it survived beautifully. The last two days all of the beautiful green, not yellow at all, have fallen off the tree.
Blue Maid Holly
Certified GKH Gardening Expert
I need a little more information. I would take a closer inspection at those leaves - at first glance they may look to be in good condition (dark green and glossy) but a look close-up may reveal paler and some darker blotching, discolored tips\edges, etc. which would yield some clues as to the cause. Are there any leaves left on the tree or did they all shed? Are there any signs of new growth on the tree?
Are there any signs of insects on the tree?
Has your holly been getting sufficient water (or too little or too much)?