Which does seem like a possibility plausible enough to be worth considering.

Quirrell put Harry on the steps, out of direct line of fire, so that Harry wouldn't try any magic to interfere with the duel or be hit by magic accidentally, and at this point he (rightfully) didn't know the Avada Kedavra could be intercepted or deflected by any magic, so he shouldn't have expected that Harry would be able to send any magic at all out into the line of fire of the spell.

I think it is mostly hindsight bias: before this particular event, nobody knew you could put magic in the way of an Avada Kedavra. Therefore, Quirrell should not have expected that Harry could put magic in the way of his Avada Kedavra, causing the reaction.