With no photos or description of the site, growing conditions, watering and fertilization, we have to guess.
If you have grass around a citrus tree then it is not "something" killing it but "someone" who wasn't aware of basic conditions for citrus trees, and it is they who have killed it.
Citrus are susceptible to "crown rot" Phytophthora cinnamomi, a fungus-like pathogen, due to over watering. The frequent watering that grass takes to keep it alive is too much for the citrus.
If the grass is dying, then the other common reason for decline is water deficit. Drying and wilting and browning occurs with drought stress.