I am not exactly sure why people are doing this, maybe it is because with excessive SEO of shorter,more competitive keywords and so people are no longer finding what they want with shorter keywords.
One word : 24.32% (0%)
Two words : 23.55% (-1%)
Three words : 20.52% (-1%)
Four words : 13.69% (-1%)
Five words : 7.94% (1%)
Six words : 4.30% (2%)
Seven words : 2.33% (3%)
Eight and more : 3.35% (6%)
Average Length of Google Search Keywords @ Tech Crunchies – Internet Statistics and Numbers