It's because somone else in the lobby has better connection to the other players in the lobby. (In noob words, Your Connection Sucks!) A way to tell if you're going to be host or not is by watching "Getting Match Quality..." Once it changes to "Migrating Hosts..." watch to see if it switches your name with someone else's in the lobby, if it does, you're not the host anymore, if it doesn't move anyone's name, you're the host.