Why destroy Laketown's bridge?

[Hot, crispy nice hobbit]

Amid shrieks and wailing and the shouts of men he came over them

Whoa, Sam, stop. Your legs are short, so stop and think! Why must the men of Lake Town chop down their path of escape?

Panic, I suppose...

"Sir! We're under attack from a flying dragon!" - Twitching guard.
"Down with da bridge! Down with da bridge!" - Screaming sergeant
"Its a Flying Dragon, Sir!" - Twitching guard
"Down with da Bridge! Down with da Bri..." - Screaming sergeant