Canonizations must be considered infallible teachings of the Church's Magisterium because

(1) their declarations are an extension of the infallibility of the Roman Pontiff due to their intimate connection with revealed dogma and the difficulties they would mire us in if they were not theologically certain, and

(2) because of their connection to the sacrifice of the Mass, which is always holy and pure, inasmuch as if canonizations could be errant it would do damage to the intrinsic holiness of the sacrifice of the Mass, something that could not occur without imperiling the holiness of the Church's sacraments, and because

(3) the Church's failure to honor the saints of other Christian communions shows that the Church regards them as somewhat dubious, which sheds light on the truth that her certainty about the blessedness of her own saints is not in any way dubious.

Thus, we can have confidence that those whom we invoke here below do indeed exist among the blessed, beholding the vision of God, and interceding for us continually.

http://www.unamsanctamcatholicam.com/theology/81-theology/74-infallability-of-canonizations.html