It may have been partly that or maybe he'd been singing wrong...
A few years back, I noticed that my stamina was absolutely shot - I'd be able to do a few songs but after that, it was all over and I was straining to do the rest of the set. That straining didn't help either so it was a vicious circle - sing wrong, strain, hurt your throat, sing wrong to try to cover up the problems, strain more, hurt your throat...
What I did was stop playing guitar altogether and just concentrate on singing from the correct place in my chest, using correct techniques and I tell you what, it made ALL the difference! I added the guitar back in after that but was also very mindful of what I was doing for ages, just so I didn't slack off and fall into bad habits...
When you're on a monster world-wide tour, you get damn tired! It's a hard job doing Bruce vocals, so you can imagine what it's like night after night after night, worse even if you have a cold but still have to go sing - that makes stuff extra tricky...
It would be soooo easy to fall into bad techniques unless you're really on the ball every night (as I said, a hard job to do on a monster tour), so I'd say that had a lot to do with it...
Lazyness? Maybe too - he was going through a weird time with Maiden then as well...
I think it's more the time he took off to reasses what he's doing plus a good long break that's gotten him back to his amazing self now!