No, it hasn't been explained yet. But if you saw the back of the bus, it was riddled with bullet holes. Somebody could have been shot as they were leaving, died, and turned. Which could explain the rest of them dying in the close quarters of the bus.
Pretty convenient way to get rid of a lot of people at once, since they were extras to the group from Woodbury.
The other thing is that some of them could have been close to death with the flu.
But here's the other side of the coin...
These folks have been through this "zombie apocolapse" for a while now. I would think if you're in the company of someone or someone(s) were close to dead or close to death, they (the remaining folks) would have pitched their arses off the bus.
I dunno, I hope they explain that a little better or at some point come across a surviving group from the bus. If we find out that every person on that bus died, I'll give that part of the show a thumbs-down.
The three worst parts of episode 10 for me were the "Carol reveal", bus thing and Tyreese, instead of helping out more at the train-track massacre decides to spend most of his time hitting one zombie in the head, over and over and over again... while other folks are getting bit.