@ondiz
My two cents,
removing the aphids may be the main contributor here. Aphids produce honeydew that the sooty mold loves, and that inhibits the photosynthesis.

Especially if it is an old tree with a big root system and doesn't usually have a severe lack of water, I'd say mulching may have helped, but in a very light way, especially after only a bit of time.

However, if the mulch was rich in nutrients and it sat there for long enough (at least 6-9 months), giving them the time to get in the soil, than it may be a main contributor and the tree was likely in a lack of something