They wanted to make it look like they have more games than they actually have at launch by securing timed exclusivity of Ryse and Dead Rising 3. They had neither that much time, nor resources to develop multiple proper exclusives to be ready for launch. I know it makes perfect sense. Same case for Sony. The only exception is Microsoft felt like they have some money to burn on that.
I don't get why people panic out of sudden. To be honest, the notion of exclusivity has now become a bit muddled to say the least with Sony employing exactly the same tactics as Microsoft, so in a way only Nintendo games are 100% safe in this regards. However, I don't think that many (or any) Xbone offerings will lose their exclusivity from now on. Maybe some lesser titles, like Master Chief Collection once Halo 5 has its premiere (but Halo 5 won't come to PC for sure). I wouldn't worry about it. That's my two cents.