A California plane crash in 1948 killed a number of migrant workers seeking to return to Mexico after a term of seasonal labour. Guthrie felt that their tragic deaths should have been better honoured, such as by some media reports that failed to identify the victims by name. The implication is that Mexicans were not considered equal to other Americans. A memorial was finally erected in tribute, in 2013.
Apparently, Woody only spoke the words, and music was added much later by others. Here is Woody’s son, Arlo, singing the song for us: