It was not that long ago that 10 was fighting with 13 as the ratings leader...now they are trying to fend off 8 for second/third place. Yes, no doubt about it, 10 has fallen, but this is not really that recent...it has been a steady fall over several years.
In my opinion their decline is based on two factors--programming, and personnel decisions.
NBC prime-time programming is probably worse than it has ever been--and it has been bad for several years...compare that with CBS's programming--with huge lead ins at 11. (I actually think 8's numbers should be higher than they are given the fact that their programming is so dominant--but I will leave that discussion for another day). 13 is going to always have their dedicated viewers, 8 is going to have their viewers who watch their news because they watch their programming....and 10 is left all the sudden with scraps.
As far as personnel...it was pointed out before, and I believe correctly that 10's numbers began to fall especially at 11pm when Jennifer was put on the anchor desk. She has credibility issues, and has not translated to viewers. The question becomes how far will they have to fall before a change is made there?
