The mountain never died. He took a vow of silence. There is some weird notion that he died and came back. Pretty sure atleast in the book he was just really close to dying
In the books, the Mountain died and it's heavily implied that he doesn't even have a head or at least someone else's head. Remember that the Martells receive the original Mountain's head.
And also temember that Book! Qyburn is a necromancer
There's a tinfoil theory that he's using Robb Stark's head hence the alias Robert Strong which is what Robb Stark means.