The movie might be referencing a real-life incident, but it's never explicitly stated in the film.
In the movie, the accident is portrayed as a contributing factor to Leigh Anne Tuohy's decision to take Michael in. However, the specifics of the accident are left vague.
If you're curious about Michael Oher's real life, you could look into his biography or other sources to see if there's information about a truck accident in his past.