Yellow leaves at this time of year, can only mean 'some'
problem, aka, the plant is not happy! Very hard to know
the exact cause without more DETAILS (and even then, maybe).
Possible causes are under/over watering, under/over
nutrients, lack of sunlight, insects, etc.,...
George (NJ).

ryecooder,
I looked at the pictures that you posted earlier, The trees looked good at that time, I have found that if the leafs are turning yellow from the bottom up, that would be from too much water, and if it is turning yellow at the top down, it could be not enough water.
But yellow leafs is not limited to water, or the lack of.
Hope this helps.
Cecil