Bad back could be due to many things. I'm no doctor so the following is just advice from experience.
Walking down to fast:
The walk down is not areobic and so people tend to go faster, also its the retun journey its the 'end of the day' and so despite looking forward to the walk for weeks before, once you have been out for 5-6hrs, you begin to look forward to getting back to your car / B&B which speeds up the pace.
Back pack:
All my packs have sternum straps on them. i think is the most important support on a ruck sack and it aids the shoulder straps. A waist belt is great too. Every strap should be pulled tight so that the pack doesnt move. But not so tight that the straps are cutting into you. If your pack is bouncing around on your back, it likely to cause you back pain and lower back blisters.
Back pack content:
Without going into a full list of what you should and shouldnt take out with you on a walk, i'll simply say that many 'new' walkers always take too much JUST IN CASE. This being a big problem because you have to carry the stuff on your back all day. Did you walk down with more than half the amount of food you took? How much (in litres) fluids did you take up? Of that how much did you bring down?
Did you check the weather forecast see that it was going to be a wonderful day and still pack mid layers?
List all the things you packed because you thought it was a good idea but didnt actually use...
In short, my advice would be to slow it down on the way down, be smart with your packing and food and finally, make sure your pack is securly on your back.
If you still get problems, seek more advice and/or consult your doc.
For me, i find my shins can hurt on the way down. Ive slowed it down and it hurts less but there is defo a problem there. My father has arthritus so i really should go to the doc but i'm scared he'll tell me to stop waking.
The pyg track is great, but i havent walked down it myself. 2011 is my Snowdonia year. In August i'll be renting a house down there for 10 days. A complete waste of money as i wont spend much time in the house.