From canon. Because this was the year that Quirrellmort tried to steal it out of the vault at Gringotts. So it needed to be somewhere safer—the only place safer than Gringotts → Hogwarts.
In canon, Quirrelmort tried to steal it after it was taken away. We were never told why Dumbledore suspected that Voldemort would try to get a hold of it that year.
From canon. Because this was the year that Quirrellmort tried to steal it out of the vault at Gringotts. So it needed to be somewhere safer—the only place safer than Gringotts → Hogwarts.
In canon, Quirrelmort tried to steal it after it was taken away. We were never told why Dumbledore suspected that Voldemort would try to get a hold of it that year.