Particle horizon and big bang The farthest radiation we can see is from the particle horizon. But the farthest radiation is the CMB. Why does all radiation from the particle horizon stem from that earliest possible source time? In other words, why is the CMB almost infinitely redshifted, suggesting that if we could see right back to the BB, the light would be infinitely redshifted? Infinite redshift is a feature of any light coming from the particle horizon, so for some reason the BB is the only event on the particle horizon.
 A: 
The farthest radiation we can see is from the particle horizon. 

Not quite right. It is quite hard to see light through a plasma. Note that we can see the surface of the sun, but it is hard to see the inside (and not just because the light from the center takes so long to get out from repeated scattering, it also just is hard to image because it scattered so much). So what you see (with electromagnetic waves) is different than the particle horizon. 
The particle horizon extends out farther. It limits neutrinos and gravity waves and everything. Electromagnetic waves stop at the surface of last scattering not because that is the big bang but merely for the reason that long ago the universe was quite hot and dense and so there were enough ions to make it hard to see past it.
The CMR comes from the surface of last scattering from the epoch of recombination. The light was scattering off ions up until it started to get cool enough and undense enough that there was a chance that light can escape and travel to us without getting scattered over and over again.

But the farthest radiation is the CMB. 

There is radiation farther away. Just like there is radiation inside the sun even though we can't see it because we only see the surface. Think of it as being hard to see hot things and the universe used to be hot everywhere as opposed to now where it is only hot in some places.

Why does all radiation from the particle horizon stem from that earliest possible source time?

It doesn't. If we can detect gravitational waves we will be able to see things from before recombination we will see things older than CMR. And neutrinos are also detectable from before recombination, recombination is just about electromagnetic ions interacting with photons.
