Question

What did the crowd do that may have frightened the soldiers into thinking they might be attacked by the people in the street

Answer 1

The crowd produced an effect in the soldiers.

It is usually happens in the relation of humans. The psychology could explain it better, the feelings, thoughts and intimidation behaviour causes effects.

May be the people just wanted stay there and talk loud but nothing  else. They studied the emotional state of soldiers  and find the goal.  

In resume, the thoughts and feelings could be do things that I want.