In this series the sequence of letters is backwards and the distance between them increases:
Z (-2) -> X (left 'Y' in)
X (-3) -> U (left 'W, V' in)
U (-4) -> Q (leaves 'T, S, R' in)
Q (-5) -> L (leaving 'P, O, N, M') is -2, -3, -4, -5 in descending order.