I was browsing the U.S. President's Wikipedia page and I noticed that after Truman left office all President's from then on began taking the Oath in January. Was there special circumstances following Ike's election which then became precedent or was a law passed?
I have no idea where you might have seen that Truman was inaugurated President in March. The change was due to the 20th Amendment, which starts "The terms of the President and Vice President shall end at noon on the 20th day of January". The annotation below states why a Constitutional amendment was needed to make the change: the first presidency started on March 4, and the Constitution says that the term is 4 years, so without amendment all further terms had to start March 4.
The amendment was declared enacted on February 6, 1933, so the 74th Congress was the first to be affected (starting January 3, 1935) and the first Presidential term affected was Rooseveldt's second term (starting January 20, 1937). (Interesting trivia point: no person has ever been elected and served more than two full four-year terms.)