Steam has just recently released "Free to Play" games, i.e., games that can be downloaded and used for free via Steam. Making a flagship title like TF2 among this class of games greatly increases awareness of Steam's new free offerings. While I doubt this is the complete reason, it probably played a role.
I would also guess that Valve has something big up their sleeve to be willing to give away such an important money maker, but I have no backing for that other than instinct. Now what would be really awesome would be open-sourcing TF2.
I think we're starting to see some land grabs by bigger players in the digital distribution of games. Electronic Arts has just launched their Origins service and Stardock sold off Impulse to Gamestop. Valve is enticing users onto their platform with all of these free-to-play additions.
I would also guess that Valve has something big up their sleeve to be willing to give away such an important money maker, but I have no backing for that other than instinct. Now what would be really awesome would be open-sourcing TF2.