Dean was expecting another direct attack from the woman, not the almost diversionary tactic. So when the garbage cans became pigeons, Dean was completely surprised, as was Seamus's mum. One pigeon got her good enough to make her drop her wand. Dean managed to hold onto his longer, but only until one of them went for he eyes.
Dean manged to punch the pigeon right out of the air, but his wand went flying as he did. And neither of them hand a chance to accio it back while the birds were still attacking.