Mysterious Announcer Message ("You have ten minutes remaining to lock in.")

Something very strange happened in the game I just played. Around 17:25 , the announcer--the voice in game that says "Welcome to Summoner's Rift", "Blue team triple kill", etc.--made an announcement I had never heard before. It was something like, "You have ten minutes remaining to lock in.". Naturally, I had no clue what that message meant, and I asked my team if they heard it, but they didn't. A similar message repeated five minutes later ("...five minutes remaining...") and two minutes later ("...three minutes remaining..."). Just as oddly, nothing actually happened around 27:25 when the time ran up. Can any player or Rioter shed some information on this? I'm incredibly confused.