Yes too much water. Does that pot have a drainage hole?

Or too little water. Before watering, check to see if it needs water by digging
a couple of inches into the soil mix to see if it is damp. Alternatively, shove
a wooden skewer halfway between the tree and the wall of the pot and all the
way to the bottom. Pull it out and if you see particles on it, then it does not
need watering.

The problem may be your soil, which looks to be very water retentive and may hold too much water preventing the tree's roots from getting the oxygen that it needs.

Many people on this forum use either the
gritty or 5-1-1 mix, both of which are fast draining and do not retain too much
water.. The ingredients for 5-1-1 are easier to find. This is what I use:

My version of Al Tapla's 5-1-1 Mix

· 5
parts pine bark mulch (remove ½” pieces through 1/2 inch screen)

(Some people use Repti Bark instead of
mulch. They do not screen it.) Be sure to soak the bark overnight.

· 1
part sphagnum peat

· 1
part perlite (coarse, if you can get it)

· 1 Tb garden
lime /gal 5-1-1 mix (measure the 5-1-1 mix)

· Osmocote
plus (follow manufacturer’s instructions). This is optional.

·
Moisten well and wait at least 2 days before using

Yeah that pot has plenty of holes. It could well be the soil though. I am willing to try the 5-1-1 mix but just don't understand why citrus in their natural environment don't seem to require that type of soil.

Living in a pot is far different than living in the ground.

Potting mixes must be very porous because that's what is required to drain well in the shallow depth of a pot.