Since 9/11 and this being a major public event with runners from around the world, it's reasonable to see the use of such dogs or even drills, no?
Or maybe there was a report of terror activity. Of course anything else could also be and we'd never be told about it.
“Experience should teach us to be most on our guard to protect liberty when the Government’s purposes are beneficent. Men born to freedom are naturally alert to repel invasion of their liberty by evil-minded rulers. The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding.” — James Madison