I have a Brogdon that has been in the ground for about 8 years (maybe 15-18 feet tall). I do notice it dropping a lot of its leaves each spring when it puts out buds, and worried about it the first time, but have come to expect it.

If yours is losing "almost all" of its leaves you may have a problem, that should not happen. My suggestion: don't fertilize anymore, I have found that my tree only needs a little. Also, suggest you clear the mulch away from the trunk. I was one of those "if a little is good, a lot is better" people and had 2-3 inches of the trunk down to the ground covered with mulch. Leave the trunk bare so it does not get soaked constantly with water. See the article below on proper mulching and a mulch "volcano".

Good luck with your Brogdon. Mine does well when there is a freeze and I really like the fruit.

http://www.uvm.edu/pss/ppp/pubs/oh78mulch.htm

I just talked to Jene (of Jene's Tropicals) today about my bacon avocado and she told me it's normal when the tree is budding fruit for a lot of the leaves to drop.
However, she does not recommend mulching, and has set rules for fertilizing that I've been following successfully on my other fruit trees (see my 4-6-8 thread as I don't remember off the top of my head)

Thank you all for the helpful comments!

Those of you with dogs, be very careful growing avocados since the are very toxic to all animals even horses. Every part of them, the fruit, the leaves, the stems and bark.