Is the ash tree trunk being injected or are injections into soil?   What insecticide is being injected? The reason I ask is that one commonly-injected insecticide has little/no effect on Lilac-Ash Borer (LAB).

The swellings/bulges or "burls" in the trunk suggest that this ash tree has had some problems with LAB in the past.   LAB larvae (caterpillars) often tunnel into the sapwood, structurally weakening the tree. LAB caterpillars may also damage wood/tissue that transports water upward from roots to leaves. If much of this xylem is damaged or removed by LABfeeding, the tree may not be getting enough water to leaves. 
Then, during hot weather, affected ash trees may lose some leaves.
   
Check the lower 10 feet of the trunk for exit holes approx 1/8-1/4 inch diameter, as in photos at  
http://www.ext.colostate.edu/pubs/insect/05614.html

You may need to have the tree pruned to remove only dead wood. Try to increase the vigor of this ash tree with light fertilization next May. Water the rooting area during dry snowless periods in winter. Ash trees that are stressed for whatever reason are more attractive to LAB.    
If you find several exit holes, suggest you have the trunk sprayed with insecticide next early May, as described in fact sheet referenced above. 


Consider hiring a Certified Arborist or a Consulting Arborist to evaluate the tree 
for hazard potential (LAB-affected ash trees may become prone to wind breakage). Also consider that the near-certainty of more injury caused by Emerald Ash Borer in a few years might be a good reason to replace this ash with another type of tree now.

Yellowing and falling ash tree leaves. Could it be....

Could it be from Emerald Ash Borer?    With 99.9% confidence, no.... that is unlikely.  EAB has only been found in Boulder. 

Sorry, My full response was in the wrong box. I was not asking about emerald ash borer or any borer. I set up an appointment with a certified arborist. Many leaves have continued to fall. He also saw no current borer evidence, only much older evidence. He seemed certain that it is anthracnose, though rare. He was optimistic that the tree would recover , especially if I removed as many leaves as possible from the grass and allowed them to do an treatment (injection). So I am not pursuing replacement this year. Does that make sense?

I'd have doubts about an anthracnose (fungal disease)  diagnosis, even with all the rain we've had recently and earlier in the season. 
If it is anthracnose, keeping fallen leaves picked up will help.  What is the proposed treatment/injection?

Locally, many more tree problems and symptoms result from non-living causes than from living causes (insects, disease)

Our lawn service arborist proposed and I accepted an injection with a systemic fungicide on 9/23 to treat anthracnose. All leaves were essentially off the tree by that time. Is it possible this condition was acerbated by the three years of pointer injections for borers? A few other ash began yellowing and losing some leaves early, but none to the extent we did.For now my plan is to wait and see if we get new growth and leafing in spring rather than replace the tree. Your thoughts?

IF anthracnose was the problem, it may help if next spring is cool and wet.
Its doubtful that Pointer (imidacloprid) injections exacerbated this problem (though you should know that imidacloprid doesn't work well on lilac-ash borer (caterpillars).   It will work well on Emerald Ash Borer (beetle) if and when that becomes a problem in Arapahoe County).

Yes, wait and see about new growth next spring.   In the meantime, suggest you water the rooting area of your ash trees during warm dry snowless periods Dec-Apr.