All of our language (speech) is made up of phonemes. These are the basic sounds we make when we say words. Playing our own language in reverse, we engage "confirmation bias", as Rlyeh said, to 'hear' phonemes in what was said. We do this because we already know what we are hearing is speech. Except it's not really, it's speech in reverse.
Because we have convinced ourselves we hear phonemes, the language centre in our brain then arranges these phonemes into 'words' - because that is what our brain does. This is pareidolia. We haven't really heard speech, we have heard sounds we know were speech (although reversed) and recombined those sounds into a recognisable pattern.
Edited by Leonardo, 20 April 2012 - 12:11 PM.