Why was Wall Street shocked at the news that the Roosevelt administration had filed an antitrust suit against Northern Securities in 1902?

Investors in Wall Street did not think that the young president would dare to challenge corporate power.

Explanation:

In 1902, public demands for "trustbusting" (breaking up the monopolies) prompted him to file suit under the Sherman Act against the biggest railroad trust in the country.

News of Roosevelt's antitrust lawsuit shocked business leaders. J. P. Morgan went to the White House to meet with Roosevelt. "If we have done anything wrong," Morgan said, "send your man to my man and they can fix it up."

In the seven years he served as President, Roosevelt brought suit against 43 other trusts.