Danny Masterson is settling into his third prison in two months ... and TMZ has learned why he's been on the move so much.

Despite reports claiming concerns for Danny's "well-being" were behind his recent transfer from one of California's worst prisons to a more humane facility, we're told that's not quite the reason for the move.

Sources with direct knowledge tell TMZ ... there was no attack on Danny at Corcoran State Prison, nor were there any credible threats against the ex-"That '70s Show" star. There've been a lot of rumors/speculation that something might've happened to him -- but it's all BS.

As we reported ... Danny started the year at North Kern State Prison, a medium security detention facility in Delano, before being transferred at the end of January to the maximum security CSP, the same penitentiary that once housed the late Charles Manson.

After a couple weeks at Corcoran, Danny was transferred to the California Men's Colony in San Luis Obispo, a medium to minimum-security prison.

Like we said ... all the shuffling around he's been doing between different prisons lately had some folks wondering why Masterson was being moved around ... and now we know.

As it turns out ... we're told North Kern is just a reception center where inmates are processed into the California prison system ... so it's literally a starting point, and wasn't ever going to be his permanent facility.

Our sources tell us Masterson had a primary prison and a secondary prison designating where he would be sent after being processed at North Kern ... the primary prison was California Men's Colony, and the secondary was Corcoran.

But, when it was time for Masterson to leave North Kern, we're told there was no space for him at CMC ... so he was sent to the backup option -- the infamous CSP. Unclear if these were prisons Masterson himself chose, or if it's just where officials put him down on paper.

His latest move was explained to us this way -- after a couple months at maximum-security Corcoran, we're told there was an opening at California Men's Colony ... so the California Department of Corrections and Rehabilitation sent Masterson there, since that's where he was supposed to be all along.

So there you have it -- he's been getting moved around because of logistics ... not for any other reason.