The fact that two unrelated species have died indicates it is not likely a disease or insect related issue, rather a cultural and/or climate related one. It is also unlikely planting the pecans had anything to do with the death of the first pine.  

Stress is most likely the issue as we have had numerous weather events that have weakened and stressed many plants. Just in the past three years we had a deep freeze, high heat and drought, including a very dry winter with another quick freeze. These all lead to stress which can also lead to insect and/or disease issues taking advantage of the weakened state of the plant. The pool water does have chemicals in it that could also lead to problems on some plants. I don't know if a soil test would be helpful to determine if there are any high salts or other related chemicals that could lead to stress, but if you haven't had a soil test done in a long time then it wouldn't hurt. The Oklahoma County OSU Extension Center can assist with a soil test. Other factors to consider are soil compaction, soil moisture, damage from lawn mowers and string trimmers, herbicide applications, etc. 

The pine in the images with brown needles is the natural shedding of leaves that occurs this time of year. Needles that are two or more years old naturally drop from pine trees. From what I can see the newest leaves look healthy. If you are still concerned about the trees a closer look/inspection might be needed. A certified arborist or other professional could help you assess the health and situation of the trees. You can find a certified arborist at this web site - https://www.treesaregood.org/. The OSU county extension center can also provide additional assistance.

Oklahoma County OSU Extension Center
2500 NE 63rd St.
Oklahoma City, OK 73111
Monday - Friday: 8 a.m. - 4:30 p.m.
Phone: <personal data hidden>, https://extension.okstate.edu/county/oklahoma/horticulture/