Speaking in a post on the game's forums, Riot explained that a handful of players have been "attacking our live service to kill games that are in-progress."
Given the popularity of the game, this sort of behavior is sadly expected. Said a producer, "They're doing this through a form of Distributed Denial of Service attack and not a hack. As of yesterday's hotfix, we've put some changes in place that prevent games from dying. Players in a targeted game will still experience lag and possibly be disconnected, but the client should now be able to reconnect to the game automatically and resume where it left off."
"To spell this out a little, if your client suddenly lags and presents you with a "Connecting screen" you should allow it to try its magic and reconnect you to the game. When it is successful this will allow you to rejoin the battle much quicker without reloading the client."
Steps have be taken to "identify these attackers" which will be removed from the service permanently. The UK's Police and Justice Act of 2006 specifically outlawed DDoS attacks, setting a maximum penalty of 10 years in prison. Likewise in the US, DDoS attacks can constitute a federal crime under the Computer Fraud and Abuse Act, and can also carry penalties of serving time in prison.
This sort of thing is unfortunately not a first for League of Legends. Earlier this summer, several European League of Legends players were informed that their accounts were hacked. Luckily, Riot Games reported that "Absolutely no" billing or payment information was breached.
"Additionally we are pursuing legal avenues and working with the proper authorities," said Riot. "We're doing everything we can to implement short- and long-term solutions to fight these types of activities. When someone's intentionally ruining the experience of other players, we take it very seriously."