Hi,I have killed several citrus trees in similar fashion.The
older leaves have turned yellow and dropped from the bottom
up.Do they leave the little stalk left on the tree attached
to the trunk? Thats what always happened to me.I have grown
to realise that it is too much water that causes that.I have
also found adding a couple of teaspoons of epsom salts each
time you feed helps the plant take up the nitrogen and greens
your leaves.Water deeply and leave to almost dry out,the roots
need air,thats what killed my trees before i reckon.
You can use warm water with a little washing liquid for the
compacted rootball you have.Water with slightly warm water
each time.
I recently bought a new orange tree and put it in a very free draining container mix.I also have tried this superthrive on advice from others here.My tree looks greener,it was a bit yellow on top.Do the leaves curl up from the sides in a u shape? Thats too much water.I water every 9 days and its summer here.Do the stick your finger down 3 inches bit,if it comes out dry water, if soil is sticky dont water.
Good luck,there should be plenty here who know more than me.cheers

Just another point,what size pot was it in before when you bought it? That was another factor,everyone here reckons to have 2 inches of space around the rootball in your new pot.I did a similar thing,went from a 10 inch pot to a huge one!!
the soil just stayed soggy.It pays to go up to a slightly
bigger pot than the excisting one.Two inches all around.The
soil is slow to dry out otherwise,leaving the roots soggy.
cheers

And that makes the root susceptable to root rot very easily.

Massive blooming everywhere is a sign of severe stress. As if the plant is making a final effort to reproduce itself before dieing!
As others have said, probably due to root rot from soggy conditions. Could even be too soggy round the edges but still dry in the middle.
What did you plant it in? You need a very free draining compost. People on this forum frequently recommend Coconut Husk Chips as the basis for good potting. I would carefully wash off all soil/compost, inspect roots and re-pot.

I agree repotting into a smaller pot..Go up 1-2 sizes of the previous pot..a huge pot in NOT the answer..it makes matters worse. Leaves soil soggy until plant dies.
I'd also refrain from fertilizing. For one, after u repot, you should wait a couple months before feeding..and secondly, if a plant is ailing food should be withheld altogether. Superthrive may help, depending how bad off your tree is..If it's rotted, well, I doubt it'll come back, but don't toss..Wait until spring to see if new growth sprouts.
I think you did too much as once..Since it was a new plant, perhaps waiting to repot or feed would have helped. Most nurseries feed before shipping plants off, usually using pellets that last 3 months.
You also want to use a well-draining soil. I use all purpose, sand, peat and perlite. You want to use something that drains when you water. Now where water sits on top of soil..The soil is too heavy if this is happening.
Good luck, Toni

Thanks for all the input. I think I will let the soil dry out for a while and see if the tree makes a recovery in spring. The remaining leaves are deep green and healthy so perhaps it just needs to dry out. I'm pretty sure the soil drains alright. I think, as all of you said, that I tried to put too small of a plant into a large pot and the deep part of the pot just doesnt get dry enough between waterings.

And also aside from above, check the pH of your soil. I had a very same problem with my Meyer last year. I found out that the soil was too acid. And I also found out that Meyer is the most tantrumatic citrus of all. So what I did was I drenched the soil (run the water overnight tricle only)with water hose to get rid of the acid and the salts in the soil.