Round won't start after countdown

Sometimes i end up in a lobby where the countdown timer starts from 30 seconds, counting down to 00, but the round wont start. I keep waiting and waiting, sometimes i have to wait up to an extra 60 seconds before the round actually starts, and then some players have already left so it’s just the Predator and me left in the game, sometimes there is one more Fireteam player still there.
What is causing this?