the meaning of 'drag one's mind back in time'

[kjc0123]

What is the meaning of 'dragged his mind back in time' in the following sentences?

Looking up at the stone towers of Saint-Sulpice, Silas fought that familiar undertow... that force that often dragged his mind back in time, locking him once again in the prison that had been his world as a young man. The memories of purgatory came as they always did, like a tempest to his senses... the reek of rotting cabbage, the stench of death, human urine and feces. The cries of hopelessness against the howling wind of the Pyrenees and the soft sobs of forgotten men.

[woofgang]

he's remembering something that he doesn't want to remember