It does sound like something happened in the second running. Two culprits to consider are PH (as mentioned) and water temp. The temp thing is easy: just keep it below 76C.
The PH thing is a little more complicated, since you have to know the PH of the mash as Jeepinjeepin explained. I highly recommend you follow his advice and get PH paper before you try this next time.
You basically need to know two things: The PH of the mash (important!) and the PH of the sparge water (not as important, but a lot easier to get). The PH of the mash is a little tricky, because you wont know it until you do it. So what you can do is this: Take a PH of the first runnings. If you are fly sparging, then take a reading towards the end of the sparge. This will give you an indication of where your mash is at present. Theoretically, you should keep it between 5.2 and 5.8, with anything below 6.4 considered "OK".
Now here's the trick: The more you sparge your grain, the more the PH will tend to climb, and the more tanins will get extracted. In order to prevent that from happening, you need to acidify the mash. You can do that by modifying the mash itself (by adding acidulated malt) or by using low PH sparge water. The benefit of the second method is that it is easier to adjust.
Ideally your sparge water should be between PH 5.2 and 5.4. You can achive this by adding food grade phosphoric acid or lactic acid to your water. I know that about half a tbsp of 80% Phosphoric acid is enough to acidify about 5g of my water. Your water may have a different acidity than mine, so my suggession is that you add a little bit (half a tsp) of acid at a time and keep taking PH readings until you hit the right range. Keep in mind: most PH paper is designed to work at room temp, so if you dip it directly in your HLT you'll get a wrong reading. Instead, take a small sample (smaller the better, since it will cool faster) and measure that. Once you've reached the desired range you can go ahead and sparge. Again, as you did with the water, you should take a couple of PH readings in a small room-temperature sample, just to make sure that the overall PH doesn't go over 5.8. If it does, add a little bit more acid to the water.
I know this sounds like a hassle, but after you do this once or twice you will have the process dialed-in, and then you can take just two measurements (one of the water, one of the mash) just to verify everything works. The upshot is that if you do get this process down, you can produce a whole other batch from the same grain (you can even produce completely different beers by "capping" or adding more grains to the mash before the second sparge) Partygail is great if you do it right!
Good luck
