Great to hear the problem is abating.
Water is one possible cause. Iron deficiency can cause chlorosis (yellow leaves). Iron is one of the nutrients that is made less available by increasing pH. In most areas the water authority doses water to keep pH a bit above neutral (pH7) to stop water pipes deteriorating. In some areas the water is naturally high pH so can come through the taps at pH 8 or 9.
We can counteract high Ph water for plants by using acidifying fertilizer. I believe significant areas of USA water supply is alkaline (high pH) so Miracid is an readily available fertilizer that helps counteract the water and bring pH back down closer to neutral (pH7).
Otherwise adding extra iron (iron chelates, iron sulphate, etc) is an alternative. Other nutrients can also be limited by high pH so you need to investigate all the ramifications of pH nutrient lock up if that's an issue.
You should be able to check local water authority testing on their website to find out a whole lot of info about the water you get.
Fertilizing the lawn is possible, but not likely, depending what he used. Usually lawn fertilizer will be high N and should make your trees grow faster. Some bonsai growers use lawn fert in the development stage to help trees grow faster.