Wow. Talk about making a somewhat difficult subject to understand subject more complicated than it needs to be.
All Va really is is a relationship between an angle of attack at a given airspeed and the Critical angle of attack. That airspeed is the one at which a full and abrupt control movement will cause a stall rather than cause damage to the airframe. Hence the definition of Va. Pretty much what has been said in the podcasts.
However, in my opinion, the discussion goes downhill from there into a technical world that is way more detailed than it needs to be.
One must understand that the difference between critical angle of attack and VA angle of attack is a constant, in other words, the relationship does not change. One also must understand the effects of weight and airspeed on angle of attack. When those two things are understood, then one is about 95% of the way towards understanding WHY Va has to decrease with decreased weight.
So, as weight decreases, what happens to Angle of attack at a constant airspeed? It decreases also, since less lift is needed. Since we understand that the difference in Va AoA and Critical AoA is a constant, what must we do to regain that difference? We need to INCREASE our angle of attack. How do we do that? We slow down, since we know that AoA is a function of both airspeed AND weight.
So, if asked on an oral why Va decreases as weight decreases, I propose the following answer: (Assuming it has already been discussed what Va is.)
"The difference between Va AoA and the Critical AoA is a constant. At given airspeed, as the weight decreases, the angle of attack decreases also. But since we are working with a constant, we need to make sure the AoA remains constant. To do that, we must decrease our airspeed, since we know that as we decrease our airspeed, AoA must be increased for a given weight. That is how we maintain the constant AoA."
It may not be all that eloquent, but I think it is more understandable than Jason's explanation on the podcasts.