A Patronus (incorporeal!) intercepting an Avada Kedavra spell is bizarre and ridiculous. Anything less ridiculous would not have been able to interfere. With hindsight, sure, he should have known, but he doesn't have hindsight, we do. Before the event, he couldn't have possibly planned for Harry interfering like this, and we can see he already planned for any more probable level of interference - making Harry lie down on the steps far away, ordering him not to get involved, etc. If you really honestly think he should have expected a smart eleven-year-old boy possessed of a stronger-than-usual Patronus charm to be able to deflect the undeflectable curse, then he also should have planned for some equally bizarre event such as Dumbledore breaking the Apparation wards on Azkaban in order to teleport in front of the Avada Kedavra spell, and just taking it on chin because he's invincible.

Honestly. Quirrell would have to be holding either the Idiot Ball or Batman's belt in order to prepare for this.