Originally published on ProWrestlingSheet.

Lucha Underground star Big Ryck will NOT be appearing on the show's upcoming season ... and sources tell Pro Wrestling Sheet his reason for dropping out is still a mystery.

Ryck -- aka former WWE Superstar Ezekial Jackson -- was prominently featured in LU's first season, but we're told he recently informed producers he wouldn't be returning this month when they film.

Sources close to the wrestler say he has yet to specify his reason for splitting with Lucha Underground, and those around him were surprised by the decision. It's unclear if he was forced to sign any sort of non-compete clause.

Earlier this month, Big Ryck's rep released a statement saying rumors about a possible retirement were false -- so it doesn't sound like he's taking a break from the ring.

Luckily, sources tell us two of the wrestler's associated with Ryck on the show are safe -- Willie Mack and Shawn Daivari. They're both currently booked for the upcoming set of tapings.

The plot thickens.