Well, the brine shrimp could have been infected... were they live?
But remember, even if you float the bag in the tank, they can die of pH shock if you don't acclimate them to the actual water. After floating for 15-20 minutes, you'll need to start adding small doses of tank water into the bag every 5-10 minutes for a couple of hours.
Then, net them out (don't dump) into the water. That way you can prevent that from happening.
Another thing is, how long did you leave them in the bag? They say to try not to leave them in the bag without opening it for more than maybe 20-30 minutes, as the fish can actually run out of fresh oxygen in the water.