could be form lack of feeding. although this suggesstion is quite debatable. in the wild these corals dun get much food, but they do catch some. torch corals have strong prey capturing ability, as u can see by their long and powerful tentacles. so they can and will accept small pieces of food like mysis and prawn. prolong starvation can cause recession.
maybe u can check ur lightings too? if the lighting is not enough, u can supplement LPS with food. without food and light, the LPS will receed and die.