twitter/bootstrap — pull request #1549 by TannerNMO (2012-02-01 21:16:22+00:00)
Title: Fixed simple grammatical errors. 

http://grammar.quickanddirtytips.com/backward-versus-backwards.aspx

Backwards is never used as an adjective

More info:

Backwards is only used as an adverb in the United Kingdom.
If in the United States, only backward should be used, at all times.