They either boo'd or luu'd. Either way, it's pretty disrespectful to Lack, and I can't see how he would not take that personally. Those people weren't sending a message to the organization.
With that said, I still think they should have started Luongo. They must have known that this would cause another controversy, especially considering how big the game was. I get wanting to win, and playing the hot hand, but in a game like this, you need to play Luongo. The only reason to sit Luongo is if he's injured or being traded, but it's been three games, and I doubt they would sit him for that long just so he doesn't get injured.