Hmmmm hard to say - I had just cleaned the bike really well the day before and changed the brake pads since they were very worn. After I changed the pads I certainly did need to adjust them, but I usually do since I adjust them in as the pads wear. I never even braked with the new pads as I wasn't allowed to ride before the VO2 and we opened the brakes completely when we put it on the trainer.
The day before my rim failed had been a particularly gritty day. It wasn't raining, but it was damp outside so all of the sand that was out on the roads still was sticking to the bikes in a lovely gritty slurry. It would have been better if it had actually been raining since then at least then the grit would have been getting washed away.
I did a little poking around on the internet. It looks like I'm not alone with having a Deep V fail suddenly and catastrophicaly. The theory that's thrown up mostly is that since the walls of the rims are taller than most that they don't have to wear as much to collapse (plus apparently deep V's are pretty thin too). I had just filled the tires the night before also. Since I'm little I generally go with about 100/110psi, even though the tires are rated to 120, but because I was having the test and its better to have the tires very solid when you are on a computrainer (less slippage I think) I did actually put 120psi in them, which may have also contributed. When the tire heated on the trainer - bam! - now I get teased by my coach about my awesome rim destroying power (I was up to a whole 190 watts when it went.....)