It became effective after the crash but was passed before. Economics is influenced by what people believe is going to happen in the future.
You're mistaken. The stock market crashed on October 29, 1929 and Smoot-Hawley gained final passage in the Senate on June 13, 1930 by a slim 44 to 42 margin. Following is an excerpt from http://www.eh.net/encyclopedia/?article=obrien.hawley-smoot.tariff/A>:
The Republicans did well in the 1928 election, picking up 30 seats in the House -- giving them a 267 to 167 majority -- and seven seats in the Senate -- giving them a 56 to 39 majority. Hoover easily defeated the Democratic presidential candidate, New York Governor Al Smith, capturing 58 percent of the popular vote and 444 of 531 votes in the Electoral College. Hoover took office on March 4, 1929 and immediately called a special session of Congress to convene on April 15 for the purpose of raising duties on agricultural products. Once the session began it became clear, however, that the Republican Congressional leadership had in mind much more sweeping tariff increases.
The House concluded its work relatively quickly and passed a bill on May 28 by a vote of 264 to 147. The bill faced a considerably more difficult time in the Senate. A block of Progressive Republicans, representing midwestern and western states, held the balance of power in the Senate. Some of these Senators had supported the third-party candidacy of Wisconsin Senator Robert LaFollette during the 1924 presidential election and they were much less protectionist than the Republican Party as a whole. It proved impossible to put together a majority in the Senate to pass the bill and the special session ended in November 1929 without a bill being passed.
By the time Congress reconvened the following spring the Great Depression was well underway. Economists date the onset of the Great Depression to the cyclical peak of August 1929, although the stock market crash of October 1929 is the more traditional beginning. By the spring of 1930 it was already clear that the downturn would be severe. The impact of the Depression helped to secure the final few votes necessary to put together a slim majority in the Senate in favor of passage of the bill. Final passage in the Senate took place on June 13, 1930 by a vote of 44 to 42. Final passage took place in the House the following day by a vote of 245 to 177. The vote was largely on party lines. Republicans in the House voted 230 to 27 in favor of final passage. Ten of the 27 Republicans voting no were Progressives from Wisconsin and Minnesota. Democrats voted 150 to 15 against final passage. Ten of the 15 Democrats voting for final passage were from Louisiana or Florida and represented citrus or sugar interests that received significant new protection under the bill.